Inhibition of in vitro natural killer activity by the third component of complement: role for the C3a fragment.

Abstract

Purified human native third component of complement, C3, was found to inhibit in vitro natural killer (NK) cell cytotoxicity in both mouse and human systems. The effect was dose and time dependent, a 50% inhibition being reached with 190 nM C3 (35 ,ug/ml) added during the NK assay or after a 30-min preincubation of the effector cells with this C3 concentration. C3 was shown to act at the effector-cell population level because pretreatment of the target cells did not modify the NK lysis. The inhibition was not due to general cytotoxicity nor to cell agglutination. Moreover, another in vitro cytotoxicity system (represented by alloreactive cytotoxic lymphocytes) was not affected by purified C3. Structural analysis ofthe active part ofthe C3 molecule shows that the C3-induced inhibition is supported by the C3a fragment. Release of carboxyl-terminal arginine residue by carboxypeptidase B, converting C3a into des-Arg7-C3a, did not alter the inhibitory effect displayed by this fragment. These results suggest that C3a may play an important role in the regulation of NK activity.Natural killer (NK) cells are defined as spontaneously cytotoxic lymphocytes with broad target specificity mainly directed against tumor cells and virus-infected cell lines (1, 2), but the mechanisms controlling their cytotoxicity level are still poorly understood. In this regard, it has been shown that NK cell cytotoxicity can be strongly increased by interferons (IFNs) released during a viral infection (IFN-a) (3) or during the activation of T cells (IFN-'y) (4) and also by interleukin-2 (5). On the other hand, macrophages from mice injected with polyanions have been reported to exert suppressive effects upon NK activity (6, 7). Noteworthy findings are that macrophages are well known to express receptors for the native third component ofcomplement, C3, on their cell membrane (8) and also to synthesize C3 molecules (9). Moreover, bacteria and polyanions are activators of the alternative pathway of the complement in which C3 is the pivotal protein and a precursor of several fragments (10). Some of the C3 fragments have properties enabling reaction with cell surfaces through either acceptor sites (as for C3b) (11) or receptor sites (as for C3a, C3b, iC3b, and C3d) (8-12), and various biological functions have been associated with these fragments. Thus, C3a is an anaphylatoxin (13), and C3b (or iC3b and C3d) can mediate attachment of target particles and soluble complexes to macrophages and to B cells that express specific membrane receptors for these fragments, promoting in this way their specialized cell functions (14).Such data prompted us to investigate the possible functional relationship between C3 and NK activity. In this report, we show that highly purified human C3 has an inhibitory effect on in vitro mouse and human NK activities. This inhibition is dose and time dependent, acts at the effector-cell population level, and is carried by a small fragment of C3, the C3a. MATERIALS AND METHODSEffector Cells. Mouse spleen cells were ha...

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