IgM-bearing B cell affinity subpopulations possess differential antigen sensitivity in rainbow trout

The need for accurate assessments of in vitro generated antibody prompted examination of the effect of antigen on secreted antibody concentrations and affinities. It was found that the antigen concentrations commonly employed for in vitro stimulation were able to significantly compromise IgM titer and affinity estimates in rainbow trout. Specifically, IgM titers were underestimated with the high affinity antibodies being specifically blocked. To remedy this, pulsed antigen cultures were employed, and it was found to reveal more accurate IgM antibody titers and affinity estimates. Additionally, pulsed dose responses provided evidence that high antigen concentrations specifically suppressed high affinity B cell induction. Optimal expression of high affinity antibodies required exposure to lower concentrations of antigen. Each affinity subpopulation appeared to possess a graded sensitivity to each dose of antigen, revealing the complex dynamic for differential IgM-bearing B cell induction that is possible during a response. These results reveal not only the need for antigen removal prior to in vitro antibody secretion, but also the possible role of high zone immunological tolerance on IgM affinity maturation in rainbow trout.

The Use of Bacteriophages and Immunological Monitoring for the Treatment of a Case of Chronic Septicemic Cutaneous Ulcerative Disease in a Loggerhead Sea Turtle Caretta caretta

In this case study, phage therapy was applied to treat a multidrug-resistant case of septicemic cutaneous ulcerative disease (SCUD) caused by Citrobacter freundii in a loggerhead sea turtle Caretta caretta. Phages were applied topically, intravenously, into the carapace, and into the exhibit water using various phage cocktails specific to the causative agent over an 8-month period. This was performed in conjunction with antimicrobial therapy. The animal was monitored through weekly cultures, photographs, and complete blood cell counts, as well as immune assays (phagocytosis, plasma lysozyme and superoxide dismutase activity, and plasma electrophoresis profiles). The animal, in comparison to an untreated, unaffected control, had elevated antibody titers to the administered phages, which persisted for at least 35 weeks. Although cultures were clear of C. freundii after phage treatment, the infection did return over time and immune assays confirmed deficiencies when compared to a healthy loggerhead sea turtle. Immune parameters with statistically significant changes over the study period included the following: decreased phagocytosis, increased alpha- and gamma-globulin protein components, and an increased albumin : globulin ratio. When C. freundii appeared again, the multidrug-resistant status had reverted back to normal susceptibility patterns. Although not completely known whether it was another subspecies of bacteria, the therapy did resolve the multidrug-resistant challenge. Phage therapy in combination with antimicrobial agents may be an effective treatment for sea turtles with normally functioning immune systems or less-severe infections. Additional research is needed to better understand and quantify sea turtle immunology.

Immune parameters in the intestine of wild and reared unvaccinated and vaccinated Atlantic salmon (Salmo salar L.)

Forming a barrier to the outside world, the gut mucosa faces the challenge of absorbing nutrients and fluids while initiating immune reactions towards potential pathogens. As a continuation to our previous publication focusing on the regional intestinal morphology in wild caught post smolt and spawning Atlantic salmon, we here investigate selected immune parameters and compare wild, reared unvaccinated and vaccinated post smolts. We observed highest transcript levels for most immune-related genes in vaccinated post smolts followed by reared unvaccinated and finally wild post smolts, indicating that farming conditions like commercial feed and vaccination might contribute to a more alerted immune system in the gut. In all groups, higher levels of immune transcripts were observed in the second segment of mid-intestine and in the posterior segment. In the life stages and conditions investigated here, we found no indication of a previously suggested population of intestinal T cells expressing MHC class II nor RAG1 expression.

CCR7 is mainly expressed in teleost gills, where it defines an IgD+IgM- B lymphocyte subset

Chemokine receptor CCR7, the receptor for both CCL19 and CCL21 chemokines, regulates the recruitment and clustering of circulating leukocytes to secondary lymphoid tissues, such as lymph nodes and Peyer's patches. Even though teleost fish do not have either of these secondary lymphoid structures, we have recently reported a homolog to CCR7 in rainbow trout (Oncorhynchus mykiss). In the present work, we have studied the distribution of leukocytes bearing extracellular CCR7 in naive adult tissues by flow cytometry, observing that among the different leukocyte populations, the highest numbers of cells with membrane (mem)CCR7 were recorded in the gill (7.5 ± 2% CCR7(+) cells). In comparison, head kidney, spleen, thymus, intestine, and peripheral blood possessed <5% CCR7(+) cells. When CCR7 was studied at early developmental stages, we detected a progressive increase in gene expression and protein CCR7 levels in the gills throughout development. Surprisingly, the majority of the CCR7(+) cells in the gills were not myeloid cells and did not express membrane CD8, IgM, nor IgT, but expressed IgD on the cell surface. In fact, most IgD(+) cells in the gills expressed CCR7. Intriguingly, the IgD(+)CCR7(+) population did not coexpress memIgM. Finally, when trout were bath challenged with viral hemorrhagic septicemia virus, the number of CCR7(+) cells significantly decreased in the gills while significantly increased in head kidney. These results provide evidence of the presence of a novel memIgD(+)memIgM(-) B lymphocyte subset in trout that expresses memCCR7 and responds to viral infections. Similarities with IgD(+)IgM(-) subsets in mammals are discussed.

The structure of rainbow trout IgM influences antibody-mediated phagocytosis of bacteria (160.16)

The tetrameric IgM molecule is the dominant immunoglobulin found in the serum of fish. In contrast to the fully polymerized mammalian IgM, teleost IgM is variably disulfide cross-linked resulting in considerable structural diversity. Recently, it was revealed that following vaccination, rainbow trout alter the assembly of their antigen-specific IgM. It was hypothesized that these different IgM forms may allow this molecule to switch effector functions. To investigate whether there was a link between structure and function, we used serum with differing IgM structural profiles and evaluated their ability to mediate phagocytosis. We discovered that when IgM was fully disulfide cross-linked it increased phagocytosis of the antibody-opsonized bacteria. Conversely, when a larger percentage of the IgM was held together by electrostatic forces, phagocytosis was diminished. Control experiments proved that differences in antibody coating were not responsible for the differences in phagocytic potential of the antibody pools. Thus, rainbow trout IgM that is more heavily disulfide polymerized facilitates antibody-mediated phagocytosis more effectively. To our knowledge, this is the first time that IgM structural diversity has been demonstrated to affect a biological function critical for the protection against infectious disease.

The identification and expression of a new light chain isotype, Igλ, in rainbow trout, Oncorhynchus mykiss (160.17)

The discovery of a trout EST bearing similarity to Igλ prompted an investigation of this previously uncharacterized isotype. Cloning and sequencing of gene-specific mRNA allowed the identification of two products. While the smaller product represented a fully spliced form, containing a variable segment (V), the joining segment (J), the constant region (C), and the 3’ UTR, the larger product was not fully spliced and contained a V segment, a 194bp intron, and a spliced J-C segment followed by the 3’ UTR. Phylogenetic analysis of the constant region confirmed that this new antibody isotype clustered with Igλ sequences of other species. Analysis of the recombination signaling sequences revealed that like Igλ in other species, the V and J segments are in the same transcriptional orientation with the V segment is followed by a heptamer-23bp spacer-nonamer while the J segment is preceded by a nonamer-12bp spacer-heptamer. In contrast to the Igκ and Igσ isotypes in trout that possess multiple VJC clusters, genomic analysis suggest that only a single Igλ VJC segment exists. Igλ accounts for only 1% to 3% of the total light chain expressions (IgκLC1 ~70%; IgσLC2 ~20%; IgκLC3~3%; IgλLC4~2%). Our identification of Igλ in rainbow trout shows that it is conserved throughout the teleost lineage. However, the role of the various Igλ splicing forms and whether the three Ig Heavy isotypes (IgM, IgD, IgT) demonstrate preferential light chain utilization still requires investigation.

Secretory IgD has an evolutionarily conserved role in respiratory mucosal defense (160.18)

Recently, we discovered secreted IgD in rainbow trout, which is produced by a novel exon-splicing mechanism. Molecular and cellular analysis revealed that secIgD plasma cells were distributed within the primary and secondary immune tissues. The concentration of serum IgD (2-80µg/ml) was ~400x lower than serum IgM (800-9000µg/ml). Two dominant secIgD heavy chain isoforms were identified in serum of trout which were associated with antibody light chains to form 240 or 370 kDa IgD-monomers. Antigen-specific secIgD was produced in response to vaccination with either TI or TD antigens. Interestingly, the distribution of antigen-specific IgD plasma cells within mucosal tissues revealed that they were preferentially located in the gills of rainbow trout. Analysis of gill mucosal secretions revealed that secIgD was the dominant antibody isotype, which contrasts the profile observed in serum. However, our data suggests that the number of IgD plasma cells in the gills of rainbow trout could not account for the elevated IgD concentration observed in the gill mucus. We hypothesize that serum IgD is preferentially transported across the gill epithelium to neutralize pathogens at this important site. The receptor responsible for trans-epithelial transport of IgD has proved elusive. However, the analogous distribution of IgD plasma cells and secreted IgD in the respiratory mucosa of trout and mammals would suggest that IgD provides an evolutionarily conserved function within this tissue.

A holistic view of the dynamisms of teleost IgM: a case study of Streptococcus iniae vaccinated rainbow trout (Oncorhynchus mykiss)

To date, little is known about how trout IgM, the primary antibody of fish, varies in titer, specificity, disulfide cross-linking, and affinity following immunization with a pathogen. Work using defined antigens has demonstrated that the disulfide cross-linking structure of IgM becomes increasingly more polymerized during an immune response, coinciding with an increase in affinity, but it is unknown if this has relevance to aquatic pathogens. Understanding how IgM varies following vaccination with an aquatic pathogen is of considerable importance as effector functions allocated to multiple antibody isotypes in mammals are essentially relegated to this single molecule. To gain insights into the dynamism of IgM, rainbow trout were immunized with Streptococcus iniae and individual serum titers, their specificity and affinity to S. iniae, and the disulfide cross-linking pattern of both total-serum and specific Ig were analyzed over a period of 37 weeks. We found that in vaccinated animals titer increased by a factor of ≈100 from starting levels, affinity increased 10-fold, and diversity of S. iniae proteins recognized by trout antibody increased at least 5-fold. Most intriguing, though less cross-linked IgM predominated early in response, by week 5, the fully tetramerized antibody comprised 50% of total specific protein. We propose that this is a mechanism to optimize efficacy of carrying out effector functions and recognizing a wide array of epitopes with higher affinity.

Characterization of an anti-rainbow trout (Oncorhynchus mykiss) CD3ε monoclonal antibody

This study characterizes a monoclonal antibody (mAb) produced against the cytoplasmic tail region of the epsilon chain of the CD3 (CD3ε) transmembrane protein found on T lymphocytes of rainbow trout (Oncorhynchus mykiss). Flow cytometry and fluorescent microscopy conducted on trout leukocytes with the anti-trout CD3ε mAb showed a distinctive population of IgM(-) CD3e(+) lymphocytes fitting the expected profile of T-cells. Immunoprecipitation of lysates derived from trout lymphocytes revealed a 19 kDa protein and peptide analysis confirmed its specificity for CD3ɛ. In vitro proliferation assays with T-cell mitogens, ConA and PHA, resulted in a 3 fold increase in the percentage of CD3ɛ+ lymphocytes compared to LPS and control cultures. The mAb characterized in this study will be useful in further elucidation for both the role and distribution of T lymphocytes in the teleost immune system.

Transduction of binding affinity by B lymphocytes: a new dimension in immunological regulation

To date, immunologists have operated with two primary paradigms governing the antibody response: (1) that affinity maturation is primarily dependent upon antigen-driven selection of both the germline and somatically amended repertoires, and (2) that antibody effector function is isotypically determined. The teleost model now suggests that these classical paradigms should be broadened to incorporate the ability of the B cell to transduce the strength of antigen recognition (affinity) into structural modifications of its antibody product, which, in turn, modulates the antibody's effector function. Although this relationship, thus far, has only been examined and demonstrated in the teleost, we find a number of the individual elements of this structural/functional relationship have been reported for mammalian IgM, which prompts future investigations into its universality. In sum, these findings suggest a heretofore unrecognized feature of B lymphocyte affinity discrimination, which transduces the affinity of antigen recognition into functionally modified antibodies.

Transducing the affinity of antigen recognition into graded structural modifications of the antibody product (43.21)

We have found that the affinity of B cell interaction with its cognate antigen governs the degree of post-translational modification of the IgM product in trout. Affinity-based, antigen-driven lymphocytic selection and induction by the antigen, TNP-LPS, not only induced high affinity antibodies in vitro, but these antibodies also possessed a greater degree of disulfide polymerization. Affinity purification of high affinity vs. low affinity serum antibodies also simultaneously resulted in the partitioning of highly polymerized antibodies into a high affinity fraction, while the lightly polymerized antibodies partitioned into the low affinity fraction. The in vivo functional relevance of this distinctive post-translational modification was then revealed upon the transfer of labeled antibodies to naïve animals. These latter studies demonstrated that highly polymerized antibodies possessed longer half-lives. Finally, the degree of intermonomeric disulfide polymerization occurring at the C-terminal Cys 578, appears proportional to the concentration of available mannosyl residues, ostensibly at the proximal N-linked glycosylation site to Cys 578. This latter finding suggests mechanism(s) by which polymerization and half-life can be regulated via the affinity of BCR recognition. This research was supported by NIFA (National Institute of Food and Agriculture) Grant Nos. 2002-35204-11685, 2005-01594-16271 and VIMS intramural support.

The third dimension of ELISPOTs: quantifying antibody secretion from individual plasma cells

The enzyme-linked immunospot assay (ELISPOT) is a technique widely used to enumerate the number of immune cells secreting a specific protein, such as antibodies or cytokines. A limitation with the ELISPOT assay is that it can only be used to detect a single protein of interest. Recently, the ELISPOT technique has been modified to use fluorophores allowing multiple secreted proteins to be detected simultaneously. This technique has greatly enhanced the ability to identify cells secreting multiple proteins, but has not been used to its fullest potential. We wished to accurately quantify the expression of antigen-specific antibody from a single plasma cell and to determine whether plasma cells recovered from different locations had different secretion rates. To achieve this we analyzed fluorospot images quantitatively using Mira MX 7 UL Astronomy software, and coupled this data with a quantitative ELISA to determine secretion rates from individual cells. Using this technique we were able to determine that plasma cells recovered from the peripheral blood secreted the most antibody (1.667 ng/cell/12 h) while splenic antibody secreting cells the least (0.399 ng/cell/12 h). We were able to quantify a 150 fold difference in antibody secretion between cells, with most plasma cells divided into two groups, low secretors (<0.1 ng/cell) or high secretors (>2 ng/cell). We believe this technique will be particularly useful for examining the secretion ratio of two proteins secreted from an individual cell, allowing us to determine if secretion is fixed or variable.

Unique processes of antibody assembly and systemic regulation enhance affinity maturation in salmonids (90.15)

The salmonid humoral immune response is mediated primarily by the induction of a single antibody isotype, an 800 kDa tetrameric molecule. However, this antibody molecule possesses considerable structural diversity due to non-uniform disulfide cross-linking of its monomeric subunits. The potential association of antibody affinity with the degree of inter-monomeric disulfide bonding was examined. Three distinctly different routes were taken to examine this relationship; 1) fractionation and analysis of high and low affinity antibodies from individual immune sera by differential immunoadsorption, 2) selective induction of high vs. mixed affinity antibodies in vitro from the same population of cells, and 3) analysis of total serum antibodies undergoing affinity maturation in vivo. All three methods demonstrated a direct relationship between affinity and the degree of disulfide polymerization, with the most cross-linked forms possessing the highest affinity. Utilizing the transfer of labeled immunopurified antibody to naive hosts we have discovered that the persistence of these structural isomers in vivo is also regulated, with the preferential removal of the more reduced forms from circulation. Thus it appears that control of intramolecular disulfide polymerization of the antibody molecule combined with the selective removal of lower affinity antibodies from the circulation, is critical to achieve a mature antibody response in salmonid species.

B cell heterogeneity in the teleost kidney: evidence for a maturation gradient from anterior to posterior kidney

The fish immune system is quite different from the mammalian system because the anterior kidney forms the main site for hematopoiesis in this species. Using transcription factor-specific Abs derived from the murine system, together with anti-trout Ig Abs and Percoll gradient separation, we analyzed B cells from trout kidney sections and compared them to those from spleen and blood. For this study, immune cells were separated by Percoll gradients, and the resulting subpopulations were defined based on expression of B cell-specific transcription factors Pax-5 and B lymphocyte-induced maturation protein-1, as well as proliferative and Ig-secreting properties. Comparison of kidney, blood, and spleen B cell subsets suggest that 1) the anterior kidney contains mostly proliferating B cell precursors and plasma cells; 2) posterior kidney houses significant populations of (partially) activated B cells and plasmablasts; and 3) trout blood contains resting, non-Ig-secreting cells and lacks plasma cells. After LPS induction of resting B cells in vitro, the kidney and spleen have a high capacity for the generation of plasma cells, whereas the blood has virtually none. Our results indicate that trout B cell subsets are profoundly different among blood, anterior kidney, posterior kidney, and spleen. We hypothesize that developing B cells mature in the anterior side of the kidney and then migrate to sites of activation, either the spleen or the posterior kidney. Lastly, our data support the notion that the trout kidney is a complex, multifunctional immune organ with the potential to support both hemopoiesis as well as humoral immune activation.

Experimental bacteriophage-mediated virulence in strains of Vibrio harveyi

Vibriosis is a major disease problem in prawn aquaculture. Until now there has been no clear explanation why some strains of Vibrio are pathogenic, while others are not. This study demonstrated that the presence of the bacteriophage V. harveyi myovirus like (VHML) may confer virulence to V. harveyi Strain 642. This was demonstrated by infecting naïve avirulent V. harveyi Strains 12, 20, 45 and 645 with the bacteriophage and converting them into virulent strains. The previously naïve strains of Vibrio infected with Bacteriophage VHML from V. harveyi Strain 642 demonstrated up-regulation of haemolysin, up-regulation of protein excretion, additional proteins which were recognised as toxic proteins from Strain 642 by monoclonal antibodies specific to the exotoxin sub-units, and a significant increase in mortality of larval Penaeus monodon. It was concluded that Bacteriophage VHML conferred virulence to V. harveyi Strains 12, 20, 45 and 645 and that Bacteriophage VHML either fully or partly confers virulence in V. harveyi Strain 642.

A newly developed ELISA showing the effect of environmental stress on levels of hsp86 in Cherax quadricarinatus and Penaeus monodon

The induction of hsps by stress in Cherax quadricarinatus and Penaeus monodon was investigated using SDS-PAGE, Western blotting and ELISA techniques. Western blotting showed the presence of an immuno-reactive protein to mouse alpha-human hsp70 IgG1 monoclonal antibody at a mass of 86 kDa (hsp86) in pleopod samples but was not sensitive enough to detect differences in response to stress. An enzyme-linked immunosorbent assay (ELISA) was developed using this antibody for the detection of hsp86 in the pleopods of C. quadricarinatus and P. monodon. Using this assay, significantly higher levels of hsp86 were detected in hyperthermally stressed C. quadricarinatus (21 to 70 g) and P. monodon (14 to 32 g) and hypoosmotically stressed P. monodon (14 to 32 g). Male C. quadricarinatus and P. monodon were thermally stressed with an increase in temperature from 24 to 33 degrees C for a period of 2 h then a recovery period of 6 h. SDS-PAGE gels of thermally stressed C. quadricarinatus and P. monodon samples revealed an increase in protein band intensity at 97 kDa (C. quadricarinatus) and 43 and 35 kDa in P. monodon. A 25 kDa mass protein was induced in C. quadricarinatus when thermally stressed. P. monodon were osmotically stressed with a decrease from 31 to 15 ppt for 2 h with a recovery of 6 h. SDS-PAGE gels revealed increased intensity of bands at 35 and 43 kDa and a 100 kDa band was induced demonstrating a wide range response of protein profile to stress in these species. SDS-PAGE gels of both species investigated also revealed an apparent reduction in band intensity of the haemocyanin subunits in stressed samples. The ELISA described here constitutes the first quantitative assay for the detection of a hsp in crustaceans and the following investigations are believed to be the first to describe the response of hsps to stress in C. quadricarinatus and P. monodon. In doing so, they provide a sound basis for future studies of the role of hsps in physiological functions in commercially cultured crustaceans.