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The disposition of teleost memory and plasma cells (PCs) has essentially been unexplored. As the organization of the teleost immune system differs significantly from that of mammals (i.e. no bone marrow or lymph nodes, hematopoietic anterior kidney), this disposition could be essential in understanding how comparable functions are achieved. To address this question, the primary and secondary antibody-secreting cell, B memory cell, and antibody responses to T-independent and T-dependent antigens were analyzed in trout. Although the TI and TD antibody responses did not differ substantively from one another, the secondary responses to both were significantly prolonged compared with the primary responses. Logarithmic increases in titer and affinity were achieved for both antigens during the primary, with only modest increases during the secondary response. Antibody-secreting cells, both PCs and plasmablasts, quantitatively paralleled antibody production, with antibody-secreting cells skewing to the hematopoietic anterior kidney for both antigens. However, the enhanced antigen-inducible response of immune fish (indicative of the memory pool) skewed to the peripheral blood and spleen. This pattern of memory versus PC disposition parallels that observed in mammals even though the organization of the respective immune systems differs considerably. Keywords: Anterior kidney r Memory cells r Plasma cells r Trout IntroductionTeleost memory has been defined by the development of a differentially heightened responsive state to antigen, albeit lacking in a number of mammalian features. Primary immunization can, thus, lead to increased immunogen sensitivity [1], enhanced in vivo [2][3][4][5][6] and in vitro [6][7][8] antibody responsiveness, expansion of antigen-sensitive precursor pools [9] and, most importantly, anamestic responses to pathogens [4,5,10,11] and immunoprophylaxis [4,[12][13][14]. Anamestic features absent from teleost responses include isotype switching [15] (although they possess multiple isotypes [16]), relatively modest somatic mutaCorrespondence: Dr. Stephen L. Kaattari e-mail: kaattari@vims.edu tion [17][18][19] and affinity maturation [20][21][22] as compared with the IgG responses of mammals. Intriguingly, characteristics of teleost memory B cells (MBCs) [9] parallel those associated with a class of mammalian MBCs. Recent evidence reveals that human MBCs can include unswitched, mutated IgM + cells [23]; nonmutated, CD27 + IgM + cells [24], those induced by T-independent (TI) antigens [25,26], preimmune diversified repertoires [27] (as has recently been posed for the catfish [19]), or arise independently of GCs [27,28]. Together, these previous studies prompt a renewed focus on the critical elements of μ specific memory, both in teleosts and mammals. For example, perhaps the requisites for exceptionally high-titered, extensively mutated, high-affinity responses in * These authors contributed equally to this work. [32][33][34], which home to their respective hematopoietic tissues, even though the organs ...

The momentous discovery of phagocytic activity in teleost B cells has caused a dramatic paradigm shift from the belief that phagocytosis is performed mainly by professional phagocytes derived from common myeloid progenitor cells, such as macrophages/monocytes, neutrophils, and dendritic cells. Recent advances on phagocytic B cells and their microbicidal ability in teleost fish position B cells at the crossroads, bridging innate with adaptive immunity. Most importantly, an increasing body of experimental evidence demonstrates that, in both teleosts and mammals, phagocytic B cells can recognize, take up, and destroy particulate antigens and then present those processed antigens to CD4 + T cells to elicit adaptive immune responses and that the phagocytosis is mediated by pattern recognition receptors and involves multiple cytokines. Thus, current findings collectively indicate that teleost phagocytic B cells, as well as their counterpart mammalian B1-B cells, can be considered one kind of professional phagocyte. The aim of this review is to summarize recent advances regarding teleost phagocytic B cells, with a particular focus on the recognizing receptors and modulating mechanisms of phagocytic B cells and the process of antigen presentation for T-cell activation. We also attempt to provide new insights into the adaptive evolution of the teleost fish phagocytic B cell on the basis of its innate and adaptive roles.

The induction of variable disulfide polymerization of IgM in the trout (Oncorhynchus mykiss) and its effect on its half-life were examined. An association between greater Ab affinity and increased disulfide polymerization was first indicated by the observation of this increased IgM disulfide polymerization during the process of affinity maturation. A direct association between Ab affinity and disulfide polymerization was then established by the fractionation of individual sera into high- and low-affinity subpopulations, which also resulted in the partitioning of high and low degrees of disulfide polymerization. The ability of high-affinity B cells to produce more highly polymerized Abs upon Ag induction was demonstrated by in vitro Ag-driven selection. Low Ag concentrations, which elicited only high-affinity Abs, also possessed the highest degree of polymerization, whereas higher concentrations of Ag elicited a broader array of Ab affinities, yielding a lower average affinity and degree of polymerization. Half-life studies revealed that the high-affinity, highly polymerized Abs possessed longer half-lives than the lower-affinity, lightly polymerized Abs. Finally, although the affinity for Ag is associated with elevated levels of polymerization, analysis of naive Ig revealed that the degree of polymerization alone, not affinity, appears sufficient to prolong Ig half-life.

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