Increased interleukin-10 mRNA expression in tumor-bearing or persistently lymphocytotic animals infected with bovine leukemia virus.

Interleukin-10 (IL-10), produced by Th2 helper T cells, B cells, and macrophages, can inhibit cytokine production by Th1 cells and enhance B-cell proliferation and differentiation. Here, we show that peripheral blood mononuclear cells (PBMCs) from bovine leukemia virus-infected animals with late-stage disease express considerably more IL-10 mRNA than animals that are not infected or that are in the early stages of disease. In contrast, the quantities of type 1 cytokines, IL-2 and gamma interferon, decrease with disease progression. In addition, we observed that IL-10 is expressed principally by monocytes/macrophages, not B lymphocytes, in persistently lymphocytotic animals. This observation supports a role for monocytes/macrophages in progression of bovine leukemia virus infection and, of importance, indicates that proliferating B cells are not the source of IL-10 expression. These findings suggest that IL-10 produced by monocytes/macrophages may influence the progression of bovine leukosis in animals that develop persistent lymphocytosis of B cells or B-cell lymphosarcoma.