Gr-1+ myeloid cells lacking T cell protein tyrosine phosphatase inhibit lymphocyte proliferation by an IFN-γ- and nitric oxide-dependent mechanism

The T cell protein tyrosine phosphatase is involved in the immune-system regulation, as evidenced by defective function and development of several hemopoietic cell populations in T cell protein tyrosine phosphatase (TC-PTP)-deficient mice. In particular, B and T cell proliferation is greatly inhibited when total splenocytes are stimulated by LPS or anti-CD3 mAb. To define the functional defect of TC-PTP-/- lymphocytes, we isolated T and B cells from the spleen of TC-PTP-/- mice. We show that the proliferative response of lymphocytes was greatly increased when cultured as a purified population, indicating that an inhibitory population is present in TC-PTP-/- spleen. However, TC-PTP-/- lymphocytes have a 2- to 3-fold lower proliferation rate compared with TC-PTP+/+ lymphocytes, suggesting that, as shown previously in embryonic fibroblasts, TC-PTP is involved in the control of cell cycle in lymphocytes. We have characterized phenotypically and functionally the inhibitory population present in the spleen of TC-PTP-/- mice. We show that a Gr-1(+)-enriched cell population isolated from TC-PTP-/- mice suppresses the CD3-induced proliferation of T cells in coculture in vitro. The specific inhibition of NO synthesis with N-G-monomethyl-L-arginine.monoacetate restored splenocyte responses, and there is a strict correlation between NO levels and the degree of suppression. Neutralization of IFN-gamma with specific mAb almost completely abolished the inhibitory activity of Gr-1(+) cells and concomitantly high levels of NO secretion. Moreover, inhibition of lymphocyte proliferative responses required cell-cell contact to achieve sufficient levels of NO. These findings demonstrate an important function of TC-PTP in the induction of the NO pathway that mediates inhibition of T cell proliferation.