To explore mechanisms of T cell regulation governing mucosal IgA immune response, concanavalin A-induced cloned T cell lines from Peyer's patches (PP) as well as spleen were established. The cloned cell lines expressed Thy- 1.2(+), Lyt-l(+)2(-) and were radioresistant (1,500 rad). The capacity of the cloned T cells to regulate Ig synthesis was determined by measuring their effect on lipopolysaccharide (LPS)-induced polyclonal Ig synthesis by PP B cells. In initial studies Ig secreted by B cells was determined by double antibody radioimmunoassay. LPS in the absence of cloned T cells induced abundant amounts of IgM (average 8,860 ng/2 × 10(5) B cells) and IgG (average 1,190 ng/2 × 10(5) B cells), but little or no IgA. The addition of PP cloned T cells markedly suppressed production of IgM (88 percent at the highest T/B cell ratio, 4:1), but the addition of spleen cloned T cells suppressed only a little or not at all. IgG production was inhibited by both PP and spleen T clone cells (70 percent at the 4:1 T/B ratio), wheras IgA synthesis was enhanced by both clones, but only to a limited degree.

In subsequent studies the expression of class-specific surface Ig (sIg) and cytoplasmic Ig (cIg) on/in unseparated PP B cells as well as Ig class- specific PP B cells and spleen B cells during culture with or without the cloned T cells was determined by immunofluorescence. The major findings were as follows: (a) Compared with unseparated B cell cultures and cultures of purified sIgM B cells derived from PP containing LPS alone, cultures containing LPS and PP cloned T cells showed a marked decrease in cIgM-, sIgG-, and cIgG-expressing cells that was accompanied by a striking increase in sIgA-bearing, but not cIgA-containing, cells. In contrast, unseparated B cell cultures and cultures of purified sIgM B cells derived from PP containing LPS and spleen cloned T cells did not show any increase in sIgA- bearing cells. (b) Compared with purified sIgG-bearing PP B cell cultures containing LPS alone, purified sIgG-bearing PP B cell cultures containing both LPS and PP cloned T cells showed no substantial change in sIgG- or cIgG- expressing cells, and no sIgA- or cIgA- expressing cells appeared. (c) Compared with sIgA-bearing PP B cell cultures containing LPS alone, purified sIgA-bearing PP B cell cultures containing both LPS and PP cloned T cells showed no increased proliferation, and cIgA cells did not occur. Cultures of purified sIgM B cells derived from spleen containing LPS and PP cloned T cells showed qualitatively similar changes. From these results we conclude that PP cloned T cells induced class-specific switching from sIgM- to sIgA- bearing B cells, whereas spleen cloned T cells lacked this property, although they may have induced an IgM {arrow} IgG or intersubclass IgG switch. These processes seem to be in part tissue dependent. Furthermore, the PP switch T cells appear to operate as true switch cells, which govern the pathway of DNA recombination events, rather than as classical helper cells, which act to expand already differentiated cells. Finally, these switch T cells probably account for the fact that PP are an important source of IgA B cells and also a major site of IgA heavy chain class switching during gut-associated mucosal B cell proliferation and differentation.

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