Duck antibody responses to keyhole limpet haemocyanin, human immunoglobulin G and the trinitrophenyl hapten. Evidence of affinity maturation

450 million years in the making: mapping the evolutionary foundations of germinal centers

Germinal centers (GCs) are distinct microanatomical structures that form in the secondary lymphoid organs of endothermic vertebrates (i.e., mammals and some birds). Within GCs, B cells undergo a Darwinian selection process to identify clones which can respond to pathogen insult as well as affinity mature the B cell repertoire. The GC response ultimately generates memory B cells and bone marrow plasma cells which facilitate humoral immunological memory, the basis for successful vaccination programs. GCs have not been observed in the secondary lymphoid organs of ectothermic jawed vertebrates (i.e., fishes, reptiles, and amphibians). However, abundant research over the past decades has indicated these organisms can produce antigen specific B cell responses and some degree of affinity maturation. This review examines data demonstrating that the fundamentals of B cell selection may be more conserved across vertebrate phylogeny than previously anticipated. Further, research in both conventional mammalian model systems and comparative models raises the question of what evolutionary benefit GCs provide endotherms if they are seemingly unnecessary for generating the basic functional components of jawed vertebrate humoral adaptive immune responses.

The Immunoglobulins: New Insights, Implications, and Applications

Immunoglobulins (Igs), as one of the hallmarks of adaptive immunity, first arose approximately 500 million years ago with the emergence of jawed vertebrates. Two events stand out in the evolutionary history of Igs from cartilaginous fish to mammals: (a) the diversification of Ig heavy chain (IgH) genes, resulting in Ig isotypes or subclasses associated with novel functions, and (b) the diversification of genetic and structural strategies, leading to the creation of the antibody repertoire we know today. This review first gives an overview of the IgH isotypes identified in jawed vertebrates to date and then highlights the implications or applications of five new recent discoveries arising from comparative studies of Igs derived from different vertebrate species.

Affinity maturation occurs in channel catfish (Ictalurus punctaus) following immunization with a T-cell dependent antigen

Affinity maturation of the antibody response, a process of antibody affinity increasing over response, is one of the key features of the mammalian immune system. However, the process is incompletely understood in teleost, including channel catfish (Ictalurus punctaus). In this study, IgM affinity maturation in channel catfish was investigated by estimating the kinetics of antibody affinity using ELISA and ELISPOT assays. Fish were immunized with a T-cell dependent antigen (TNP-KLH), and individual serum IgM antibody titers and affinities, and IgM⁺ antibody-secreting cells (ASCs) in peripheral blood were analyzed over a period of 14 weeks. A detectable serum anti-TNP response developed by 2-weeks post-immunization, and the maximal antibody production was observed by 6-weeks post-immunization. The average affinity of anti-TNP serum antibody increased consistently and reached the maximum by 10-weeks post-immunization. The increase of antibody affinity beyond the point of optimal antibody titer revealed that the affinity maturation of IgM antibody response occurred in channel catfish. Dissection of dynamics of individual affinity subpopulations indicated that a significant proportion of low affinity subpopulations appeared at early response, and high affinity subpopulations appeared predominantly at later, resulting in a 100-fold increase in affinity over response. Additional, TNP⁺ IgM⁺ ASCs was detected by 2-weeks post-immunization and achieved the maximal number by 6-weeks post-immunization. Using an inhibition ELISPOT assay, the findings of a consistent increase in the average affinity of secreted IgM antibody by peripheral blood ASCs, as the immune response progressed, confirmed the occurrence of the affinity maturation. Taken together, the results of this study indicated that affinity maturation occurred in channel catfish following immunization with a TD antigen TNP-KLH.

Virologic and Immunologic Characteristics in Mature Ducks with Acute Duck Hepatitis A Virus 1 Infection

Duck hepatitis A virus 1 (DHAV-1) infection in mature ducks has previously been proposed as a small-animal model for human hepatitis A. However, basic research on the outcome of DHAV-1 infection in mature ducks is limited. Here, we examined the course of viremia, the characteristics of antibody responses, and the profiles of plasma cytokines in mature ducks infected with DHAV-1. During the course of infection, the viremia was detectable soon after infection and persisted for 196 days, however, the ducks presented as clinically asymptomatic. Specific and timely immunoglobulin G (IgG), IgM, and IgA1 responses were elicited. At the same time, extensive inhibition of viral replication was observed with increasing IgG concentration. With respect to pattern-recognition receptors, TLR-7 was mainly involved in triggering the innate defense against the DHAV-1 infection. In addition, plasma immune analytes were measured and were determined to have bidirectional roles in virus clearance. It was concluded that DHAV-1 spreads quickly in blood. The spontaneous clearance of DHAV-1 during asymptomatic infection in mature ducks depends on the cooperation of timely antibody responses and alert innate immune responses. Moreover, the delayed clearance may be associated with a weak interferon-γ-producing CD8+ T cell response. This study allows us to reveal the mechanism of clearance and persistence of DHAV-1 infection in mature ducks. We anticipate that it will provide a basis for future studies focused on defining the nature mechanisms involved in the clearance and persistence of human hepatitis virus.

Immunoglobulin genetics and antibody responses to influenza in ducks

The role of the duck as the natural host and reservoir of influenza and efforts to vaccinate ducks during recent outbreaks of avian influenza has renewed interest in the duck antibody response. Ducks have unique antibody structures and expression, with consequences for their function. Aspects of immunoglobulin genetics, gene expression, and antibody function will be reviewed in the context of the duck immune response to influenza. Ducks have three immunoglobulin isotypes, IgM, IgA and IgY in translocon arrangement. The order of heavy chain genes in the locus is unusual, IGHM, IGHA and IGHY, with IGHA in inverse transcriptional orientation. IgH and IgL gene rearrangement in ducks involves limited V, (D) and J element recombination and diversity is generated by gene conversion from pseudogenes. IgY, the functional equivalent of IgG, is produced in two secreted forms, a full-length form and one lacking the third and fourth C region domains, which predominates later in the immune response and lacks the biological effector functions of IgG. The unusual features of duck antibodies may contribute to weak antibody responses and the perpetuation of the virus in this animal reservoir.

Past or present? Relative contributions of developmental and adult conditions to adult immune function and coloration in mallard ducks (Anas platyrhynchos)

Developmental conditions affect adult physiological processes and phenotypic traits, including those associated with both survival and reproduction. Carotenoids are molecules that generate sexually attractive coloration, and these pigments are acquired throughout life and can affect antioxidant capacity and immunocompetence of young and old animals. However, few studies have tracked carotenoid status and condition during development and into adulthood to understand how ontogeny affects later-life health and coloration of both males and females. We reared male and female mallard ducks (Anas platyrhynchos) from hatch to adulthood, measured circulating carotenoid titers and body condition (i.e., size-adjusted body mass) throughout development, and assessed adult immune function and integumentary carotenoid-based beak and foot coloration. We found that adult immune function (wing web swelling response to phytohemagglutinin; PHA) in males was positively correlated with body condition during the growth period of development, rather than adult condition, and similarly that both male and female beak coloration was associated with developmental, rather than adult, body condition. We also found associations between coloration and health during adulthood; males with more carotenoid-rich beaks (a sexually attractive feature) tended to have a more robust adult PHA response and a greater antibody response to a novel antigen, while females with less carotenoid-rich beaks had greater antibody responsiveness at adulthood. In addition, male beak color changed over the course of the 24-h PHA test in proportion to the degree of PHA swelling. However, intensity of foot coloration (a trait of unknown sexual significance) was not associated with any condition, carotenoid, or immune metric for males or females. Taken together, our findings implicate key developmental components to the expression of both survival- and reproduction-related traits at adulthood, but that for a dynamic trait like beak color, there are also important adult conditions that can alter signal expression.

B-cells need a proper house, whereas T-cells are happy in a cave: the dependence of lymphocytes on secondary lymphoid tissues during evolution

A fundamental tenet of immunology is that adaptive immune responses are initiated in secondary lymphoid tissues. This dogma has been challenged by several recent reports. We discuss how successful T cell-mediated immunity can be initiated outside of such dedicated structures, whereas they are required for adaptive humoral immunity. This resembles an ancient immune pathway in the oldest cold-blooded vertebrates, which lack lymph nodes and sophisticated B-cell responses including optimal affinity maturation. The T-cell, however, has retained the capacity to recognize antigen in a lymph node-free environment. Besides bone marrow and lung, the liver is one organ that can potentially serve as a surrogate lymphoid organ and could represent a remnant from the time before lymph nodes developed.

Immunoglobulins of the non-galliform birds: antibody expression and repertoire in the duck

Galliform and non-galliform birds express three immunoglobulin isotypes, IgM, IgA and IgY. Beyond this we should not generalize because differences in gene organization may have functional consequences reflected in the immune response. At present, studies on non-galliform birds are largely restricted to ducks. Ducks express an alternatively spliced form of their IgY heavy chain (upsilon) gene, the IgY(DeltaFc), that lacks the Fc region and Fc-associated secondary effector functions. It is not known how common the expression of the IgY(DeltaFc) is among birds, nor the functional consequences. It is also not known whether the unusual organization of the duck IgH locus, also shared with the chicken, having the gene order of mu, alpha and upsilon, with alpha inverted in the locus, is unique to the galloanseriform lineage. Ducks, like chickens, have a single immunoglobulin light chain of the lambda (lambda) type. Evidence suggests that ducks, like chickens, generate their immunoglobulin repertoire through a single functional rearrangement of the variable (V) region, and generate diversity through gene conversion from a pool of pseudogenes. In Southern blots of germline and rearranged bursal DNA, both the heavy and light chain loci of ducks appear to each undergo one major rearrangement event. For both heavy and light chains, the functional V region element and the pseudogenes appear to consist of a single gene family. Further analysis of 26 heavy chain joining (JH) and 27 light chain JL segments shows there is use of a single J segment in ducks, which is diversified presumably through somatic mutations and gene conversion events. Despite this limitation on the rearrangement of immunoglobulin genes, analysis of 26 DH and 122 VL sequences suggests that extensive sequence diversity is generated.