Systematic characterization of porcine ileal Peyer's patch - I. Apoptosis-sensitive immature B cells are the predominant cell type

It is now apparent that the Peyer's patches of some species exhibit structu ral, functional and developmental heterogeneity. In sheep, for example, the ileal Peyer's patch (IPP) is the primary, antigen-independent site for the generation of the primary immunoglobulin repertoire and consequent product ion of the systemic B-cell pool. The pig has three distinct Peyer's patches , including an IPP, but the functional status of this organ, as primary or secondary lymphoid tissue, is not clear. Here, we have systematically chara cterized pig IPP follicular lymphocytes and show that about 90% B cells tha t are positive for surface immunoglobulin C (sIgM(+)) and express an immatu re phenotype characterized by expression of myeloid marker sWC3 (74-22-15) and two molecules recognized by IPP B-cell-specific monoclonal antibodies ( F10/4, F12/35). Extensive apoptosis in vivo and in vitro was demonstrated b y electron microscopy, immunohistology with TdT-mediated dUTP nick end, lab elling, DNA analysis and fluorescence-activated cell sorter analysis. Thus, when isolated IPP follicular cells were incubated at 37 degrees in vitro, the majority of them became apoptotic. The few that survived, however, had lost their expression of sWC3, F10/4, F12/35, but showed an increased expre ssion of sIgM and major histocompatibility complex class II indicating that such surviving cells were of a more mature phenotype. Although more T cell s were observed in porcine IPP follicles than in sheep IPP, CD3(+) cells co mprised less than 5% of the IPP follicular lymphocytes. Thus, the results c learly indicate that pig IPP is equivalent to sheep IPP.