Binding of fluid-phase complement components C3 and C3b to human lymphocytes

Abstract

A population of B-lymphocytes has receptors for the 3rd component of complement, C3. These lymphocytes may be identified by their ability to form rosettes with sheep erythrocytes coated with covalently bound fragments of complement component C3. Human tonsil lymphocytes, enriched for B-cells, form rosettes with sheep erythrocytes coated with antibody and complement components C1, C4b and C3b (EAC143b cells). Fluid-phase C3 will inhibit rosette formation between EAC143b and human tonsil lymphocytes over the same concentration range as fluid-phase C3b. C3 is not cleaved to C3b during incubation with lymphocytes or with lymphocytes and EAC143b cells. Fluid-phase 125I-labeled C3 and 125I-labeled C3b bind to lymphocytes in a specific manner. The characteristics of binding of both radioiodinated C3 and radioiodinated C3b are very similar, but the binding of C3 is again not a result of cleavage to C3b. Salicylhydroxamic acid does not inhibit binding of 125I-labeled C3 to tonsil lymphocytes at concentrations that completely inhibit binding of 125I-labeled C3 to EAC142 cells via the nascent binding site of C3b. C3 and C3b apparently share a common feature involved in binding to lymphocytes bearing receptors for the 3rd component of complement.