The Presence of Active C1 (C1‐) on Peripheral Human Lymphocytes1

Abstract

The 1st component of complement (C1) is present in an active form on the surface of washed human peripheral lymphocytes but not on platelets or erythrocytes. This active C1 (C.hivin.1) was detected by its ability to transfer to sensitized cells carrying C4, i.e., EAC4 [antibody sensitized sheep erythrocytes carrying functionally pure], forming EAC.hivin.1,4. Active C1 was also able to consume C4. Treatment of these lymphocytes with 0.02 M EDTA removed C.hivin.1. EDTA-treated lymphocytes were able to bind exogenous purified human C.hivin.1. Comparative studies with sensitized erythrocytes (EA) and EDTA treated lymphocytes showed that although fewer molecules of exogenous C1 could bind to the EDTA-treated lymphocytes than to EA, the consumption of C4 by C.hivin.1 bound to lymphocytes was significantly higher than that observed with EAC.hivin.1. When lymphocytes obtained from 2 patients with chronic lymphocytic leukemia and hypocomplementemia were tested, the release of C1, the C4 consumption and the binding of C.hivin.1 to EDTA-treated cells were highly inefficient.