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We previously demonstrated that the majority ( ≥90%) of porcine ileal Peyer's patch (IPP) follicular cells are immature B cells destined to die by apoptosis, when incubated at 37°. In this paper we approached the mechanisms responsible for positive selection of porcine IPP follicular immature B-cell selection, by screening for various cell types, cytokines and polyclonal and monoclonal antibodies for promoting the survival of IPP B cells. Of these reagents, only CD3 cross-linked purified T cells from mesenteric lymph nodes were able to rescue IPP follicular B cells from apoptosis, although polyclonal anti-IPP lymphocyte antibodies delayed apoptosis. This survival effect could be reproduced simply by incubating IPP follicular B cells with soluble and cell membrane-expressed CD 154, an observation consistent with the demonstrated presence of CD40 and CD 154 on porcine IPP follicular B cells and activated T cells, respectively. The IPP follicular B cells rescued in this manner expressed a more mature surface marker phenotype. Immunohistology and fluorescence-activated cell sorter analysis demonstrated that subpopulations of IPP follicular T cells (less than 0.5%) express CD 154. Thus, perhaps unexpectedly, CD 154 on T cells may play a role in the positive selection of immature B cells in the porcine IPP. The origin and control of the activated T cells identified within the porcine IPP remains to be investigated. [ABSTRACT FROM AUTHOR]