HUMAN T-CELLS EXPRESS SPECIFIC BINDING-SITES FOR C1Q - ROLE IN T-CELLACTIVATION AND PROLIFERATION

Although the receptor that binds to the collagen-like domain of human C1q (C1qR) is expressed on a wide variety of cell types, the presence or absence of this receptor on human T lymphocytes has been debatable. The current studies were undertaken to re-examine whether human T cel ls possess specific binding sites for C1q by using a combination of te chniques, including radioligand binding studies, flow cytometric analy sis, and epifluorescence imaging techniques. Radioligand binding studi es indicate that both peripheral T cells and the cultured T cell line, MOLT4, bind I-125-labeled C1q in a specific and apparently saturable manner, reaching equilibrium within 30 min at 37 degrees C under condi tions of subphysiologic (90 mM NaCl) ionic strength. Western blot anal ysis with anti-C1qR of membrane proteins derived from Raji and MOLT4 c ells showed an apparent single band of approximately 60 kDa under nonr educing conditions. Furthermore, when peripheral blood T cells were st imulated with 12,-o-tetradecanoyl phorbol-13-ester acetate for 5 days at 37 degrees C and assessed by FACS for their ability to bind anti-C1 qR, the mitogen-induced cells were found to bind 40 to 50% more than t heir unstimulated counterparts. In addition, both CD4(+) and CD8(+) T cells were found to bind anti-C1qR. When the cells were mitogen induce d with either 12,-o-tetradecanoyl phorbol-13-ester acetate, Con A, or PWM for 48 h in the presence or absence of 50 mu g/ml C1q then pulsed with 1 mu Ci [H-3]thymidine for 16 h at 37 degrees C, proliferation wa s significantly inhibited (40 to 80%, n = 7) as assessed by reduced [H -3]thymidine incorporation. Taken together, the data suggest that: 1) Human T cells express C1qR in which immunoblots reveal a 60-kDa single chain protein. 2) C1qR expression is up-regulated by mitogens that in duce T cell proliferation. 3) The primary ligand, C1q, induces an anti proliferative signal, which suggests that the C1qR plays a role in T c ell activation and proliferation. in addition, the data contribute to the characterization of C1qRs on cells in peripheral blood and indicat e that all cells, with the exception of erythrocytes, bear functional C1q receptors.