THYMIC B-CELLS OF PIG FETUSES AND GERM-FREE PIGS SPONTANEOUSLY PRODUCE IGM, IGG AND IGA - DETECTION BY ELISPOT METHOD

The aim of this study was to investigate spontaneous immunoglobulin pr oduction and a pattern of isotype switching by thymic B lymphocytes (T BL) as compared with cells isolated from spleen during early ontogeny using a pig model in which B-cell development is not influenced by mat ernal regulatory factors. A sensitive ELISPOT assay was therefore empl oyed to detect immunoglobulins in pig fetuses, colostrum-deprived germ -free (GF) piglets as well as conventionally (CONV) reared pigs. The f irst spontaneously immunoglobulin-secreting cells in the thymus were d etected in 67-day-old fetuses (the length of gestation period in pigs is 114 days), their number increasing during fetal ontogeny. In contra st to fetal splenic cells, which secrete exclusively IgM, fetal thymic immunoglobulin-secreting cells were determined to undergo spontaneous isotype switching to Ige and IgA. In 28-day-old GF piglets and 3-mont h-old CONV pigs the number of thymic immune globulin-secreting cells o f all isotypes was comparable to the number of thymic immunoglobulin-s ecreting cells detected in the newborn thymus. Considerable augmentati on of IgG and ISA 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 ear ly fetal ontogeny spontaneously undergoing isotype switching to IgG an d IgA; in the postnatal period the TBL population does not appear to b e influenced by external antigenic stimuli of conventional microflora.