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SUMMARYThe aim of this study was to investigate spontaneous immunoglobulin production and a pattern of isotype switching by thymic B lymphocytes (TBL) as compared with cells isolated from spleen during early ontogeny using a pig model in which B-cell development is not influenced by maternal regulatory factors. A sensitive ELISPOT assay was therefore employed to detect immunoglobulins in pig fetuses, colostrum-deprived germ-free (GF) piglets as well as conventionally (CONV) reared pigs. The first spontaneously immunoglobulin-secreting cells in the thymus were detected in 67-day-old fetuses (the length of gestation period in pigs is 114 days), their number increasing during fetal ontogeny. In contrast to fetal splenic cells, which secrete exclusively IgM, fetal thymic immunoglobulin-secreting cells were determined to undergo spontaneous isotype switching to IgG and IgA. In 28-day-old GF piglets and 3-month-old CONV pigs the number of thymic immunoglobulin-secreting cells of all isotypes was comparable to the number of thymic immunoglobulin-secreting cells detected in the newborn thymus. Considerable augmentation of IgG and IgA production by splenic immunoglobulin-secreting cells in CONV pigs was observed as compared to GF newborns and GF piglets, in which IgG-and IgA-secreting cells were detected occasionally. Our results indicate that TBL represent the first B-cell population in early fetal ontogeny spontaneously undergoing isotype switching to IgG and IgA; in the postnatal period the TBL population does not appear to be influenced by external antigenic stimuli of conventional microflora.

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Mucosal surfaces covered by a layer of epithelial cells represent the largest and most critical interface between the organism and its environment. The barrier function of mucosal surfaces is performed by the epithelial layer and immune cells present in the mucosal compartment. As recently found, epithelial cells, apart from their participation in absorptive, digestive and secretory processes perform more than a passive barrier function and are directly involved in immune processes. Besides the well known role of epithelial cells in the transfer of polymeric immunoglobulins produced by lamina propria B lymphocytes to the luminal content of mucosals (secretory Igs), these cells were found to perform various other immunological functions, to interact with other cells of the immune system and to induce an efficient inflammatory response to microbial invasion: enzymic processing of dietary antigens, expression of class I and II MHC antigens, presentation of antigens to lymphocytes, expression of adhesive molecules mediating interaction with intraepithelial lymphocytes and components of extracellular matrix, production of cytokines and probable participation in extrathymic T cell development of intraepithelial lymphocytes. All these functions were suggested to influence substantially the mucosal immune system and its response. Under immunopathological conditions, e.g. during infections and inflammatory bowel and celiac diseases, both epithelial cells and intraepithelial lymphocytes participate substantially in inflammatory reactions. Moreover, enterocytes could become a target of mucosal immune factors. Mucosal immunosurveillance function is of crucial importance in various pathological conditions but especially in the case of the most frequent malignity occurring in the intestinal compartment, i.e. colorectal carcinoma. Proper understanding of the differentiation processes and functions of epithelial cells in interaction with other components of the mucosal immune system is therefore highly desirable.

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Precolostral germfree piglets were perorally immunized with sheep erythrocytes daily, beginning on the 5th day of life. At different intervals the numbers of IgM, IgG and IgA antibody-forming cells were estimated in the spleens, lymph nodes and intestinal mucosa by the haemolytic plaque method. In the beginning of the response IgM antibody-forming cells were detected in intestinal mucosa and mesenteric lymph nodes. IgA plaque-forming cells appeared later and were predominant during the whole period in all organs tested. In all lymphatic organs tested the appearance of IgG antibody-producing cells followed that of IgA plaque-forming cells. The development of immunoglobulins and antibody of different isotypes in intestinal contents and sera was compared with the cellular kinetics found in these young animals after local stimulation.

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