Role of C3b receptors in the enhancement of interleukin-2-dependent T-cell proliferation.

The mechanism by which the complement system influences immune responses to T-cell-dependent antigens has not yet been clarified. That is why we studied the effect of the third complement component (C3) on different T-cell-dependent processes using well-defined mouse T-cell lines. While C3 did not influence the interleukin-2 (IL-2) production of the ST2/K-9 helper T-cells, the IL-2-dependent proliferation of the ST1 line was shown to be dose-dependently enhanced by C3. It is proved that neither the haemolytic activity of C3 nor the C3a fragment had any role in the process. The effect of C3 on the IL-2-dependent T-cell growth is even more enhanced (up to five-fold) when using polymerised C3. When the ST1 cell line is cultured in the presence of the cross-linked ligand, T-cells formed 80% less rosettes with red blood cells coated with antibody and mouse or human C3b. It is strongly suggested that C3--particularly when aggregated--exerts its enhancing effect on the growth of IL-2-dependent cell lines by binding to C3b receptors present on such T-cells.