Investigations in this laboratory are directed towards understanding the mechanisms by which microorganisms activate, modulate, or subvert immune systems. We postulate these events are mediated within developmental and regulatory pathways of precursors of regulatory T-cells for antibody forming cells and cytotoxic T- cells. Macrophages and natural cytotoxic (NK) cells may also be effected through modulatory effects upon their precursors of their regulatory cells. We have exploited various bacterial toxins as natural probes to facilitate an experimental delineation of mechanisms in this respect. An in vitro modular immune system (""""""""cell complementation system"""""""" involving murine spleen immunocytes) allows us to rearrange the order and relative numbers of toxin treated/or untreated cells when recombined in the immune system. Further progress in constructing and cloning perpetual cell lines possessing the phenotypes of many of the parent regulatory immunocytes is ongoing. These phenocopies of regulatory cells are used as targets of the bacterial toxins and subsequently tested for altered function. They may facilitate production of large amounts of homogeneous gene products and cells; thus, promoting continuity of ongoing experiments in using homogenous cells at the same stage of development. Suppressor-negative clones from one of our suppressor precursor clones (NBP2C2, a CD4/CD8 double-positive clone), in the presence of SPE were detected. Similar selections using Et resulted in subclones retaining parental phenotype. But, cell-free supernatants of Et treated, unfractionated NFR/N or nude mouse splenocytes supported an 80-90% recovery of suppressive activity by the SPE selected subclone. Macrophage-depletion negated this activity of supernatants. Selected clones were observed not to differ from their parent in expression of MHC haplotype and an epitope of the TCR-agn, while there were effects upon expression of IL-2R in the parent clone by SPE and to various magnitudes with TSST-l, Pt, and SE-C. The SPE selected subclone expressed markedly less CD8 than its parent. These results may explain the contrasuppressive activity of SPE and similar bacterial products.