Interaction of Rabbit Spermatozoa and Serum Complement Components

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Abstract
Unheated rabbit and human sera were found to induce acrosomal loss in rabbit spermatozoa, while similar concentrations of heated sera did not. Human serum did not induce acrosomal loss when pretreated with antiserum to complement component C8, suggesting that acrosomal loss in unheated serum is caused by the membrane attack complex of complement. Human serum complement anaphylatoxins did not induce acrosomal loss, although they are known to induce exocytosis of secretory granules in other cell types. When incubated directly in human or rabbit sera, rabbit spermatozoa fixed complement, i.e., reduced the potential hemolytic activity of the sera. Fixation was suppressed by adding EGTA [ethyleneglycol bis(.beta.-aminoethyl ether)N,N,N'',N''-tetraacetic acid] to reduce free Ca. This indicates that rabbit spermatozoa fix complement by intiating the classical pathway to complement activation. Initiation requires the presence of cell-bound Ig and the subsequent binding of complement component C1q. Ig were detected in detergent extracts of washed ejaculated spermatozoa by a solid-phase radioimmunoassay, and the binding of 125I-human C1q was detected on samples of living ejaculated spermatozoa. Seminal plasma was found to inhibit complement-induced hemolysis of erythrocytes. Apparently, in the absence of seminal plasma, spermatozoa may activate complement where it is present in the male or female tract.