Antigenic Composition

Shared antigenic composition of closely related parasite species is a challenge, particularly for nematodes, and often leads to cross-reactivity in immunological tests (Eysker and Ploeger, 2000;

From: Advances in Parasitology, 2013

Chapters and Articles

SALMONELLA | Detection by Classical Cultural Techniques

R. Miguel Amaguaña, Wallace H. Andrews, in Encyclopedia of Food Microbiology, 1999

Serological Confirmation

In the final step of the culture method, Salmonella spp. strains are characterized serologically by determining their antigenic composition. The antigens are classified as somatic (O) and flagellar (H). The somatic (O) antigens are determined by performing a slide agglutination test with somatic (O) antisera and growth from an agar culture of the food. The flagellar (H) antigens are determined by performing an agglutination test with the flagellar (H) antisera and a formalized saline infusion broth culture in a test tube. This test can also be performed using the Spicer–Edwards flagellar (H) antisera test. The test is positive if a specific somatic group (O) and Spicer–Edwards reactions are obtained in one or more TSI cultures.

Many culture methods are acceptable throughout the world for the detection of Salmonella spp. from foods; the most widely recognized and accepted of these methods are the ISO and AOACI/FDA methods. Although these culture methods are considered standard methods, advances in rapid test kit methods have proven to be a suitable alternative to the classical culture methods and have improved the microbiological testing of foods. Considering the recent advancement in rapid test kits, methods providing same-day test results seem to be within reach.

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Orthomyxoviridae

In Virus Taxonomy, 2012

Biological properties

Epidemics of respiratory disease in humans during the 20th–21st century have been caused by influenzaviruses A having the antigenic composition H1N1, H2N2 and H3N2. The pandemics of 1918, 1977 and 2009 were caused by H1N1 viruses, H2N2 caused “Asian influenza” in 1957 and in 1968 “Hong Kong influenza” was caused by an H3N2 virus. H1N2 reassortant viruses between H1N1 and H3N2 human viruses appeared in 2001 and became established, circulating viruses until 2004. Limited outbreaks of respiratory disease in humans caused by antigenically novel viruses occurred in 1976 in Fort Dix, New Jersey, when classical swine H1N1 viruses infected military recruits; and sporadic infections with swine H1N1 viruses have occurred in the intervening years. In 1997 and 2003 in Hong Kong H5N1 viruses caused outbreaks in poultry and contemporary illnesses and deaths in humans. The continued circulation of H5N1 viruses in birds has been associated with zoonotic infections of humans with H5N1 viruses, with a large proportion proving fatal. H9N2 viruses present in poultry have caused occasional illness in humans in China, first observed in 1998, and zoonotic infections have continued to be documented. Influenzaviruses A of subtype H7N7 and H3N8 (previously designated equine 1 and equine 2 viruses, respectively) cause outbreaks of respiratory disease in horses; but H7N7 virus has not been isolated from horses since the late 1970s. Influenzaviruses A (H1N1) and (H3N2) have been isolated frequently from pigs. The H1N1 viruses isolated from swine in recent years appear to be of three general categories: those closely related to classical “swine influenza” and which cause occasional human cases; those first characterized in samples collected from swine in 1979 and genetically more closely related in all gene segments to H1N1 viruses isolated in birds which have become established and cause infection among pigs in Europe and Asia; and those resembling viruses isolated from epidemics in humans since 1977. Swine H1N1 viruses have also been observed as triple reassortants with genes originating from the swine pool, the avian pool and the human pool; the “triple” reassortants with genes form three distinct gene pools were first observed in swine H3N2 viruses. H3N2 viruses from swine appear to contain HA and NA genes closely related to those from human epidemic strains. Triple gene reassortant viruses possessing the H3 HA and N2 NA from a recent human virus and other genes from a swine and/or avian virus were first identified in the North American pig population in 1998 and have been circulating since then. Infections of pigs with the A(H1N1) 2009 pandemic have been documented associated with reverse zoonosis; H1N2 viruses, distinct from those in humans, have been isolated from pigs in UK, France, Japan and the US. Influenzaviruses A (H7N7 and H4N5) have caused outbreaks in seals, with virus spread to non-respiratory tissues in this host. H7N7 viruses have been isolated from conjunctival infections of a laboratory worker and a farm worker in 1980 and 1996, respectively and from humans involved in disease control in 2003. In addition, in 2003, a human was fatally infected with a highly pathogenic avian influenza H7N7 virus. Pacific Ocean whales have reportedly been infected with type A (H1N3) virus. Other influenza subtypes have also been isolated from lungs of Atlantic Ocean whales off North America. FLUAV (H10N4 and H3N2) has caused outbreaks in mink. All subtypes of HA and NA, in many different combinations, have been identified in isolates from avian species, particularly wild aquatic birds, chickens, turkeys and ducks. Pathology in avian species varies from non-apparent infection (often involving replication in, and probable transmission via, the intestinal tract), to more severe infections (observed with subtypes H5 and H7) with spread to many tissues and high mortality rates. The structure of the HA glycoprotein, in particular the specificity of its receptor binding site and its cleavability by host protease(s), appears to be critical in determining the host range and organ tropisms of influenza viruses. The NS1 also contributes to the outcome of infection by mitigating host defense mechanisms; e.g., through anti-interferon activity. In addition, interactions between gene products determine the outcome of infection. Interspecies transmission apparently occurs in some instances without genetic reassortment (e.g., the direct transmission of H1N1 virus from swine to humans and vice versa, H3N2 virus from humans to swine, and the recent transmissions of H5N1 and H9N2 viruses from poultry to humans). In other cases, interspecies transmission may involve RNA segment reassortment in hosts infected with more than one strain of virus, each with distinct host ranges, or epidemic properties (e.g., 1968 isolates of H3N2 viruses were derived by reassortment between a human H2N2 virus and a virus containing an H3 HA). Laboratory animals that may be infected with influenzaviruses A include ferrets, mice, hamsters and guinea pigs, as well as some small primates such as squirrel monkeys.

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Cancer Immunotherapy with Vaccines and Checkpoint Blockade

Drew Pardoll, in The Molecular Basis of Cancer (Fourth Edition), 2015

Cancer Antigens—the Difference between Tumor and Self

Tumors reflect the biologic and antigenic characteristics of their tissue of origin but also differ fundamentally from their normal-cell counterparts in both antigenic composition and biologic behavior. Both these elements of cancer provide potential tumor-selective or tumor-specific antigens as potential targets for cancer vaccination specifically and antitumor immune responses in general. Genetic instability, a basic hallmark of cancer, is a primary generator of tumor-specific antigens. The most common genetic alteration in cancer is mutation arising from defects in DNA damage repair systems of the tumor cell.8-15 Recent estimates from genome-wide sequencing efforts suggest that every tumor contains a few hundred mutations in coding regions.16 In addition, deletions, amplifications, and chromosomal rearrangements can result in new genetic sequences resulting from the juxtaposition of coding sequences that are not normally contiguous in untransformed cells. The vast majority of these mutations occur in intracellular proteins, and thus the “neoantigens” they encode would not be readily targeted by antibodies. However, the major histocompatibility complex (MHC) presentation system for T-cell recognition makes peptides derived from all cellular proteins available on the cell surface as peptide MHC complexes capable of being recognized by T cells. Based on analysis of sequence motifs, it is estimated that roughly one third of the mutations identified from genome sequencing of 22 breast and colon cancers16 were capable of binding to common human leukocyte antigen (HLA) alleles based on analysis of sequence motifs.

In accordance with the original findings of Prehn,5 the vast majority of tumor-specific antigens derived from mutation as a consequence of genetic instability are unique to individual tumors. The consequence of this fact is that antigen-specific immunotherapies targeted at most truly tumor-specific antigens would by necessity be patient specific. However, there are a growing number of examples of tumor-specific mutations that are shared. The three best studied examples are the Kras codon 12 G→A (found in roughly 40% of colon cancers and more than 75% of pancreas cancers), the BrafV599E (found in roughly 70% of melanomas), and the P53 codon 249 G→T mutation (found in about 50% of hepatocellular carcinomas).17-20 As with nonshared mutations, these common tumor-specific mutations all occur in intracellular proteins and therefore require T-cell recognition of MHC-presented peptides for immune recognition. Indeed, both the Kras codon 12 G→A and the BrafV599E mutations result in “neopeptides” capable of being recognized by HLA class 1– and class II–restricted T cells.21-24

The other major difference between tumor cells and their normal counterparts derives from epigenetics.25 Global alterations in DNA methylation as well as chromatin structure in tumor cells result in dramatic shifts in gene expression. All tumors overexpress hundreds of genes relative to their normal counterparts and, in many cases, turn on genes that are normally completely silent in their normal cellular counterparts. Overexpressed genes in tumor cells represent the most commonly targeted tumor antigens by both antibodies and cellular immunotherapies. This is because, in contrast to most antigens derived from mutation, overexpressed genes are shared among many tumors of a given tissue origin or sometimes multiple tumor types. For example, mesothelin, which is targeted by T cells from vaccinated pancreatic cancer patients,26 is highly expressed in virtually all pancreatic cancers, mesotheliomas, and most ovarian cancers.27,28 Although mesothelin is expressed at low to moderate levels in the pleural mesothelium, it is not expressed at all in normal pancreatic or ovarian ductal epithelial cells.

The most dramatic examples of tumor-selective expression of epigenetically altered genes are the so-called cancer-testis antigens.29 These genes appear to be highly restricted in their expression in the adult. Many are expressed selectively in the testes of males and are not expressed at all in females. Expression in the testis appears to be restricted to germ cells, and in fact some of these genes appear to encode proteins associated with meiosis.30-32 Cancer-testis antigens therefore represent examples of widely shared tumor-selective antigens whose expression is highly restricted to tumors. Many cancer-testis antigens have been shown to be recognized by T cells from nonvaccinated and vaccinated cancer patients.29 From the standpoint of immunotherapeutic targeting, a major drawback of the cancer-testis antigens is that none appears to be necessary for the tumors’ growth or survival. Therefore, their expression appears to be purely the consequence of epigenetic instability rather than selection, and antigen-negative variants are easily selected out in the face of immunotherapeutic targeting.

A final category of tumor antigen that has received much attention encompasses tissue-specific antigens shared by tumors of similar histologic origin. Interest in this class of antigen as a tumor-selective antigen arose when melanoma-reactive T cells derived from melanoma patients were found to recognize tyrosinase, a melanocyte-specific protein required for melanin synthesis.33,34 In fact, the most commonly generated melanoma-reactive T cells from melanoma patients recognize melanocyte antigens.35,36 Although one cannot formally call tissue-specific antigens tumor-specific, they are nonetheless potentially viable targets for therapeutic T-cell responses when the tissue is dispensable (i.e., prostate cancer or melanoma).

From the standpoint of T-cell targeting, tumor antigens upregulated as a consequence of epigenetic alterations represent “self-antigens” and are therefore likely to induce some level of immune tolerance. However, it is now clear that the stringencies of immune tolerance against different self-antigens differ according to tissue distribution and normal expression level within normal cells. The mesothelin antigen described earlier is an example. In a recent set of clinical pancreatic cancer vaccine studies, mesothelin-specific T-cell responses were induced by vaccination with genetically modified pancreatic tumor cell vaccines, and induction of mesothelin-specific T cells correlated with ultimate disease outcome.37 Given that the immune system is capable of differential responsiveness determined by antigen levels, it is quite possible to imagine generating tumor-selective immune responses against antigens whose expression level in the tumor is significantly greater within normal cells in the tumor-bearing host. In addition, upregulated antigens that provide physiologically relevant growth or survival advantages to the tumor are preferred targets for any form of therapy because they are not so readily selected out.

Beyond the antigenic differences between tumor cells and normal cells, there are important immunologic consequences to the distinct biological behavior of tumor cells relative to their normal counterparts. Whereas uncontrolled growth is certainly a common biological feature of all tumors, the major pathophysiologic characteristics of malignant cancer responsible for morbidity and mortality are their ability to invade through natural tissue barriers and ultimately to metastasize. Both of these characteristics, never observed in nontransformed cells, are associated with dramatic disruption and remodeling of tissue architecture. Indeed, the tumor microenvironment is quite distinct from the microenvironment of normal tissue counterparts. One of the important consequences of tissue disruption, even when caused by noninfectious mechanisms, is the elaboration of pro-inflammatory signals. These signals, generally in the form of cytokines and chemokines, are potentially capable of naturally initiating innate and adaptive immune responses. Indeed, the level of leukocyte infiltration into the microenvironment of tumors tends to be significantly greater than the leukocyte component of their normal-tissue counterparts. Cancers are therefore constantly confronted with inflammatory responses as they invade tissues and metastasize. In some circumstances these inflammatory and immune responses can potentially eliminate a tumor—so-called immune surveillance. However, as discussed later, oncogenic pathways in the tumor appear to organize the immunologic component of the microenvironment in a fashion that not only protects the tumor from antitumor immune responses but can qualitatively shift immune responses to those that actually support and promote tumor growth. It is these elements of the cancer–immune system interaction that will be the central targets of future immunotherapeutic strategies.

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Antibody Identification

Sadiqa Karim MD, in Transfusion Medicine and Hemostasis (Second Edition), 2013

Antibody Identification Panel

Identification of an antibody to a RBC antigen requires testing the patient’s plasma against a panel of selected RBC samples (typically 8–14 reagent RBCs) with known antigenic composition for the major blood groups (Rh, Kell, Kidd, Duffy, and MNS) (Figure 20.1). Usually these reagent RBCs are obtained from commercial suppliers and the phenotypes of the reagent RBCs accompanies each panel. Panel cells are generally group O, thereby allowing the plasma of any ABO group to be tested. The reagent RBCs are selected so that if one takes all of the examples of RBCs into account, a distinctive pattern of positive and negative reactions exists for each of the many antigens (including D, C, E, c, e, M, N, S, s, P1, Lea, Leb, K, k, Fya, Fyb, Jka, and Jkb). The selected RBCs in a panel should allow for identification of single specificities of the common alloantibodies with exclusion of most others.

FIGURE 20.1. An example of an antibody identification panel in a patient with anti-C.

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Inactivated influenza vaccines

Carolyn B. Bridges, ... Nancy J. Cox, in Vaccines (Fifth Edition), 2008

Manufacture of vaccine

The requirements of national authorities for influenza vaccines generally reflect the guidelines published by the World Health Organization (WHO),159 but may include items that are specific to individual control authorities. Recommendations for the antigenic composition of influenza vaccines are made annually to ensure that current influenza vaccines are effective against recently circulating strains in both the northern and southern hemispheres.160,161 As early as 1947, and within 2 years of the introduction of the first commercial vaccines, it was recognized that antigenic changes in the HAs of influenza viruses could reduce the effectiveness of vaccines.162,163 As a result, the WHO global surveillance system was established in 1948. Laboratories from many countries participate and help determine whether significant changes have occurred in the antigenicity of the HAs of circulating influenza viruses. Surveillance has made it clear that antigenic changes occur not only by point mutations (antigenic drift), but also by way of antigenic shift (at irregular and unpredictable intervals). The WHO global influenza surveillance system has been expanded to improve the timely identification of antigenic changes necessary for updating vaccines.

Influenza vaccine viruses usually are isolates obtained through the WHO surveillance network. The WHO and various national authorities recommend use of certain strains based primarily on the antigenic characteristics of their HAs and NAs. Original isolates are passaged to develop reference strains that are distributed to manufacturers to develop seed viruses. Often the original wild-type strains grow relatively poorly in eggs, so considerable effort is expended examining several antigenically similar strains for several qualities needed to maximize virus yield during large-scale production, including their potential for development of high-growth reassortants, their growth characteristics, and their optimal incubation conditions (such as time and temperature). Since the available production time for trivalent influenza vaccine is limited by the need to distribute vaccine each autumn, the total amount of vaccine that can be produced is limited by the least productive strain. In that context the influenza B virus component of trivalent influenza vaccines is becoming a limiting feature in expanding availability of vaccines produced in eggs.

Currently, the strains used for manufacturing vaccines are isolated either in eggs or primary chick kidney cultures. This practice reflects the fact that embryonated hen's eggs have been the predominant ‘bioreactor’ for replication of influenza viruses since the first inactivated vaccines were produced in the 1940s, but it also has been suggested that replication of influenza viruses in eggs provides a partial barrier to extraneous agents that might originate in the clinical source material.

The increasing interest in using mammalian cell substrates such as MDCK or Vero cells to replicate the viruses for production of inactivated influenza virus vaccine may promote changes in the way influenza viruses are isolated by surveillance laboratories for production use. Because most influenza surveillance laboratories are not prepared to handle tissue cultures in a way necessary to prevent introduction of extrane-ous agents, a concern is that the possible introduction and amplification of extraneous agents during subsequent passages could compromise the safety of vaccines. The interest in mammalian cell systems stems from concern that the availability of eggs could be reduced by an event requiring destruction of the egg-producing hens (e.g., an outbreak of avian influenza or Newcastle disease virus), and also from concern that the HAs of viruses grown in eggs may exhibit antigenic alterations, theoretically limiting vaccine effectiveness.164–167 Therefore, strategies are being examined to implement direct isolation of influenza viruses in mammalian tissue culture to provide suitable starting seed viruses.

Regardless of the growth substrate used, the prevention of the introduction of extraneous agents is a concern. In relation to eggs, the main concern is the introduction of extraneous agents, such as avian leucosis virus, that originate in the flocks of chickens providing the eggs. In relation to mammalian tissue cultures, there are concerns that extraneous agents could be introduced from the original human host, from the tissue cultures used by the laboratory recovering the influenza virus, from the cell substrate used for manufacturing, or from one of the materials used to support the growth of the cell substrate.168–171 For inactivated vaccines produced either in eggs or mammalian tissue cultures, these concerns are reduced provided there is information to indicate that the process used to inactivate influenza viruses will inactivate other microorganisms effectively.

After replication of influenza viruses, a number of manu-facturing steps are taken to increase the concentration of the active immunizing ingredients of the vaccines (mainly the viral HAs) and to reduce other (mainly nonviral) materials. For inactivated influenza virus vaccines produced using eggs or mammalian tissue cultures, removal and reduction of egg or tissue culture proteins occurs throughout the manufacturing process, beginning with concentration of virus by means of centrifugation through a sucrose gradient or passage of the virus-containing allantoic fluid over a chromatographic column. The resulting fluids may be additionally purified by dialysis or diafiltration, and the concentration of residual egg or cell proteins is reduced further during the final formulation when the HA concentration is adjusted to achieve final target levels.

Disruption of the lipid envelope permits further purification of the viral proteins and, in particular, the HAs and NAs. These proteins form rosette-like structures in which the hydrophilic heads of the proteins are on the exterior and the hydrophobic tail portions of the molecules are buried internally. The disruption process varies in efficiency, so that HA and NA still can be attached to lipid but in pieces smaller than the original virion. Dissolution of the viral envelope and removal of additional viral components results in vaccine products with reduced reactogenicity.133–137,172,173

Inactivated influenza virus vaccines are intended for parenteral administration and must be sterile. Environmen-tal bacteria and fungi colonize eggs, and the unintentional introduction of bacteria or fungi during manufacturing steps is also possible whether manufacturing is in eggs or mammalian tissue culture. Chemical inactivation steps are used to minimize the microbial load in the raw viral harvest from the growth substrate, and careful handling and use of component reagents and buffers also facilitate inactivated vaccine sterility. However, filtration steps ensure elimination of undesirable microbes. Generally, sterility can be achieved using filters with pore size sufficient (0.22 μm or less) to exclude small bacteria such as Brevundimonas diminuta (previously named Pseudomonas dimi-nuta). Although the sterile filtration step does not eliminate endotoxins, which may have been formed previously by bacteria in the product, some of the purification steps can help to reduce endotoxin levels.174 Because endotoxins contribute to febrile responses to injected vaccines, the limits for endotoxin levels on finished product are set well below clinically established thresholds for reactions.175

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Quasispecies Dynamics in Disease Prevention and Control

Esteban Domingo, in Virus as Populations, 2016

8.2 Different Manifestations of Virus Evolution in the Prevention and Treatment of Viral Disease

Viral diseases are an important burden for human health and agriculture (Bloom and Lambert, 2003). Virus evolution, through the basic mechanisms exposed in previous chapters, can influence the two major strategies to combat viral infections: prevention by vaccination and treatment by antiviral inhibitors. In considering the design of a new antiviral vaccine, the extent of diversity in the field of the virus to be controlled is critical. The natural evolution of the virus may result in the circulation of one major antigenic type or to the co-circulation of multiple antigenic types. The vaccine composition (independently of the type of vaccine; see Section 8.3.1) must match the antigenic composition of the virus to be controlled. Hepatitis A virus (HAV) circulates as a single serotype while foot-and-mouth disease virus (FMDV) circulates as seven serotypes and diverse subtypes, and the antigenic types are unevenly distributed in different geographical locations. A monovalent vaccine made of the prevailing antigenic type of HAV should be sufficient to confer protection, while a multivalent vaccine composed of several types or subtypes is required to confer protection against FMDV, and the antigenic composition should be selected depending on the circulating viruses. This is why anti-FMD vaccines of different composition are used in different world areas at a given time, and vaccine composition must be periodically updated to maintain its efficacy. Thus, one effect of virus evolution relevant to vaccine design derives from the necessity to prepare a vaccine that mirrors the antigenic composition of the virus to be controlled. In the case of live-attenuated antiviral vaccines, the evolution of the vaccine virus while it replicates in the vaccinee is a risk factor to produce virulent derivatives.

The invasion of a susceptible host by a virus, and the ensuing viral replication, can be regarded as a step-wise process during which the virus must adapt to a series of selective pressures presented by the host, notably the immune response. The outcome can be either viral clearance (elimination of the infection) or virus survival and progression toward an acute or a persistent infection. Administration of antiviral agents is an additional selective constraint that limits viral replication. Evolutionary mechanisms may either succeed in selection of mutants resistant to the antiviral agent that will permit the infection to continue, or fail in sustaining the infection, resulting in the clearing of the virus from the organism.

Treatment planning, one of the aims of the new antiviral pharmacological interventions [motivated largely from information obtained by next generation sequencing (NGS) applied to the virus present in each infected patient] has some parallels with vaccine composition design. For vaccines, the information comes from analyses of antigenic composition of circulating viruses and for antiviral agents, the information comes from the analyses of the quasispecies composition of the virus to be controlled in the infected patient.

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Anticardiolipin Antibodies

Munther A. Khamashta, ... Maria Laura Bertolaccini, in Autoantibodies (Third Edition), 2014

Definition

Phospholipids are a class of polar lipid components of cell membranes. Phosphatidylserine, phosphatidylinositol, phosphatidic acid, and cardiolipin are negatively charged, whereas phosphatidylcholine is neutral and phosphatidylethanolamine is zwitteronic (dipolar ionic).

Cardiolipin is an anionic phospholipid, historically important as an antigen for testing reagin in syphilis serology. Currently it is part of the antigenic composition used in the VDRL tests along with lecithin and cholesterol. In 1990, two independent research groups showed that the “true” antigen for aCL binding was a phospholipid-binding protein, the so-called β2 glycoprotein I (β2GPI), rather than cardiolipin itself (see Chapter 81, “β2-Glycoprotein I Autoantibodies”).

A variety of other plasma proteins, also known as phospholipid-binding proteins, have been implicated as targets for aPL. These include prothrombin, protein C, protein S, annexin V, and kininogens. Anionic phospholipids may play an important role in vivo in the binding of autoantibodies to phospholipid-bound plasma proteins.

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The Impact of Vaccination on the Epidemiology of Infectious Diseases

Roy M. Anderson, in The Vaccine Book (Second Edition), 2016

10 Partially effective vaccines—efficacy versus duration of protection

As noted in the introduction, most of the “low hanging fruit” for vaccine development have been plucked, and the infections targeted by the majority of the currently available vaccines target pathogens where little antigenic variation exists. Influenza A is an exception, with drift and shift in antigenic composition of the circulating strains resulting in the vaccine being modified annually to match the appropriate viral strains.

Those pathogens that continue to cause a great deal of morbidity and mortality worldwide, for which we do not currently have effective vaccines, tend to be ones in which genetic variation in surface antigens is high, either resulting in a constantly moving antigenic landscape or a large number of strains with different antigenic compositions to target in any potential vaccine. To some extent, this problem has been addressed by the production of polyvalent vaccines for pneumococcal disease, HPV and rotavirus, but this raises the question of how natural selection might favor those stains not targeted in the current vaccines. This issue is addressed in a later section.

For the constantly moving antigenic targets presented by, for example, HIV-1 and Plasmodium falciparum, success in vaccine development has been limited to date. Recent results for a malaria vaccine look promising,33 but the vaccine is partially efficacious in protecting against both infection and associated morbidity, and the long term duration of protection afforded to those immunized is uncertain at present. The first malaria vaccine candidate (RTS,S/AS01) to reach phase 3 clinical testing is partially effective against clinical disease in young African children up to 4 years after vaccination.33 The results suggest that the vaccine could prevent a substantial number of cases of clinical malaria, especially in areas of high transmission. Such progress is encouraging, given the ability of the malaria parasites to generate antigenic variation in three different ways; namely, by mutation, recombination, and multiple but slightly different copies of the genes that encode for surface antigens (the so called var genes numbering approximately 60) whose expression can be switched on and off.34

Vaccines that are partially efficacious, and do not reach the target protection of 80–90% plus that most widely used childhood vaccines possess, and which vaccine developers aim for, may still be powerful public health tools in preventing infection and associated morbidity and mortality. It is not just efficacy that determines impact. The duration of protection is equally important. For a vaccine that provides protection for an average of V years where life expectancy is L years, the eradication criterion defined earlier by Eq. 1.8 must be modified to mirror the properties of the vaccine34

A relationship is plotted in Fig. 1.17 for various values of R0 and the duration of vaccine protection V, from which it can be seen that short durations of protection makes blocking transmission difficult. However, a low efficacy vaccine can be very effective if the duration of protection it offers is long.35

Figure 1.17. Protective vaccine with limited life of efficacy.

Critical proportion to be immunized, p, to block transmission as a function of Ro and the duration of protection, V (efficacy, ɛ = 1).

In the case of HIV-1 vaccines (none available at present), the situation could be very complicated if immunization does not protect against infection but acts to reduced viral load. In this sense it would be acting as an immunotherapy. In trial studies of such products, if or when they become available, many epidemiological parameters must be measured. At a bare minimum, the following should be measured: (1) of those receiving the vaccine (a single or short course of injections), the fraction who seroconvert and seem to be immunized (the apparent efficacy); (2) average duration of protection relative to average lifespan of sexual activity; (3) fraction of vaccinated individuals who, when exposed to the virus, become infected (vaccine failure rate); (4) ratio q of the infectiousness of infected vaccinated individuals relative to that of unvaccinated people; and (5) ratio of the length of the average incubation period of AIDS in infected vaccinated individuals relative to that in unvaccinated persons.35 Population outcomes from treating individuals with such immunotherapeutic products are many and varied, and will of course include perverse outcomes where immunotherapy may be beneficial to the individual but not to the community if it leads to continued low infectiousness and sustained risk behaviors.

The general issue surrounding the development of partially efficacious vaccines relates to the need in clinical trials, for potential vaccines against the more antigenically heterogeneous infectious agents, not only to measure efficacy (the fraction protected), but other properties as well. Most importantly, these include the prevention of morbidity as opposed to infection (as for the malaria vaccine RTS,S/AS01) and the duration of protection induced (Fig. 1.17).

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Neisseria gonorrhoeae (Gonorrhea)

Jeanne M. Marrazzo, Michael A. Apicella, in Mandell, Douglas, and Bennett's Principles and Practice of Infectious Diseases (Eighth Edition), 2015

Type IV Pili

Type IV pili (fimbria) are strong flexible filaments extending from the gonococcal cell surface. They can be several microns in length and 50 to 80 angstroms in width. Pili traverse the outer membrane of the gonococcus through an integral outer membrane protein known as PilQ.4 Mature pili are composed of repeating protein subunits (pilin) with a molecular weight of 19 + 2.5 kDa.5 Through the activity of the proteins PilT and possibly PilB, gonococci can depolymerize and repolymerize the pilus strand, causing the bacteria to “twitch.” The pilin subunit has regions of considerable interstrain antigenic similarity, especially near the amino terminus, but areas of extreme antigenic variability are also present.5,6 A single strain of N. gonorrhoeae is capable of producing pili with differing antigenic compositions. This has compromised the utility of pilus-based vaccines against gonorrhea. The presence or absence of pili result in varied colonial forms, which can be distinguished when N. gonorrhoeae is grown on translucent agar.7 Fresh clinical isolates initially form colony types P+ and P++ (formerly called T1 and T2), and the organisms have numerous pili extending from the cell surface (Fig. 214-2); P colonies (formerly T3 and T4) lack pili. Piliated gonococci are better able than organisms from P colonies to attach to human mucosal surfaces and are more virulent in animal and organ culture models and in human inoculation experiments than nonpiliated variants. Expression of pili is a function of the pil gene complex. A spontaneous shift between P+ or P++ colonies to P colony types, known as phase variation, occurs after 20 to 24 hours of growth in vitro. This is caused by errors in DNA replication, resulting in pilE genes that are out of frame and fail to produce functional protein.6

In addition to mediating attachment, pili contribute to resistance to killing by neutrophils. In the fallopian tube mucosa model (Fig. 214-3), pili facilitate attachment to nonciliated epithelial cells, which initiates a process of entry and transport through these cells into intercellular spaces near the basement membrane or directly into the subepithelial space while concurrently nearby ciliated mucosal cells lose their cilia and are sloughed.8 CD46 was considered to be the main pilin receptor, but the issue is currently uncertain and the identity of the pilus receptor is an area of active study. Other pilus-associated proteins are likely to be important to adhesion to host cells, particularly PilC.9,10 Other factors also mediate attachment, notably, opacity (Opa) proteins, lipo-oligosaccharide (LOS), and porins (Por).

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Platelet-Derived Microparticles

Rienk Nieuwland, Augueste Sturk, in Platelets (Second Edition), 2007

C Glycoproteins

Platelets and PMP share glycoprotein (GP) receptors, such as GPIb (CD42b; see Chapter 7), platelet–endothelial cell adhesion molecule-1 (CD31; see Chapter 11), and the integrin αIIbβ3 (GPIIb-IIIa, CD41/CD61; see Chapter 8). In addition, subpopulations of PMPs can expose activation markers, including P-selectin (CD62P; see Chapter 12),4 and may bind fibrinogen. The antigenic composition of PMP and their functions are dependent on the mechanisms underlying their release. For example, PMP released from platelets activated by collagen and thrombin expose integrin αIIbβ3 that binds fibrinogen, whereas PMP from complement C5b-9-activated platelets expose αIIbβ3, which does not bind fibrinogen.9

Recently, proteomics (see Chapter 5) was used to determine the proteome of plasma microparticles (MP).10 Because plasma was used as a source of MP, one has to bear in mind that these plasma MP are not only of platelet origin. The proteome of plasma MP (1021–1055 protein spots) clearly differed from plasma (331–370 protein spots), and 30 proteins were identified in the plasma MP proteome that had never been reported before in such analyses of human plasma.10 Additional studies will be necessary to establish the precise composition of MP from a single cell type, such as platelets.

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