THE TRYPANOSOMA-CRUZI IMMUNOSUPPRESSIVE FACTOR (TIF) TARGETS A LYMPHOCYTE-ACTIVATION EVENT SUBSEQUENT TO INCREASED INTRACELLULAR CALCIUM-ION CONCENTRATION AND TRANSLOCATION OF PROTEIN-KINASE-C BUT PREVIOUS TO CYCLIN D2 AND CDK4 MESSENGER-RNA ACCUMULATION

Many immunosuppressive effects of Trypanosoma cruzi can be reproduced in vitro by a preparation consisting of molecules spontaneously releas ed by this protozoan (termed trypanosomal immunosuppressive factor (TI F)). In this work, we attempted to establish whether TIF-induced inhib ition of lymphoproliferation results from preventing lymphocyte activa tion or impairing a post-activation process. Although [H-3]thymidine u ptake and expression of CD25 by normal human T lymphocytes stimulated with a phorbol ester were markedly reduced by T. cruzi or TIF, translo cation of cytosolic protein kinase C (PKC) to the cell membrane was no t affected. Lymphoproliferation induced by ionomycin was also inhibite d by T. cruzi or TIF but the typical elevation of intracellular calciu m ions [Ca2+](i) caused by this calcium ionophore was not altered. The increase in [Ca2+](i) induced with anti-CD3 antibody was also unaffec ted by TIF. TIF did not preclude lymphocytes stimulated with phytohema gglutinin from accumulating normal mRNA levels of NFAT1 (also known as NFATp) and NFATc. NFAT1 and NFATc are components of the NFAT complex that controls transcription of genes coding for several cytokines and whose translocation to the nucleus is dependent upon PKC activation an d increased [Ca2+](i). In contrast, the mRNA levels of cyclin D2 and c dk4, which form a holoenzyme complex known to regulate cell progressio n through the G1 phase, were markedly reduced by TIF. These results in dicated that TIF did not inhibit lymphocyte activation leading to earl y secondary signaling but curtailed a mechanism controlling cell progr ession through G1 and necessary for reaching S phase. (C) 1998 Elsevie r Science B.V. All rights reserved.