INTERLEUKIN-10 IS EXPRESSED BY BOVINE TYPE-1 HELPER, TYPE-2 HELPER, AND UNRESTRICTED PARASITE-SPECIFIC T-CELL CLONES AND INHIBITS PROLIFERATION OF ALL 3 SUBSETS IN AN ACCESSORY-CELL-DEPENDENT MANNER

Murine interleukin-10 (IL-10) is produced by type 2 helper (Th2) cells and Selectively inhibits cytokine synthesis by type 1 helper (Th1) cells, whereas human IL-10 is produced by and inhibits proliferation and cytokine synthesis by both Th1 and Th2 subsets. This study reports that bovine IL-10 mRNA is expressed by Th0, Th1, and Th2 clones of bovine T cells specific for either Babesia bovis or Fasciola hepatica but not by two CD8(+) T-cell clones. The antigen-induced proliferative responses of all three subsets of CD4(+) cells were inhibited by human IL-10, and low levels (10 U/ml) of exogenous human IL-2 restored the suppressed response. However, proliferation of one Th1 clone was never inhibited but was enhanced by IL-10. Human IL-10 also inhibited the expression of gamma interferon and IL-4 mRNA in Th0 clones. In the absence of accessory cells (AC), the responses of Th clones to concanavalin A or IL-2 were not inhibited by IL-10 whereas antigen-specific responses of Th1 and Th2 cells were reduced when IL-10-pretreated macrophages were used as AC. Together, our results with bovine T cells support the concept that IL-10 primarily affects AC function and does not directly inhibit CD4(+) T cells and demonstrate that the immunoregulatory effects of IL-10 are not selectively directed at Th1 populations, as they are in mice.