The Possible Role of Immunological Complement In Induction of Rabbit Sperm Acrosome Reaction

The rabbit acrosome reaction, which involves membrane breakdown and release of enzymes for penetration of egg investments, is induced by a specific stimulus in follicular fluid at the site of fertilization. Since follicular fluid is a blood plasma filtrate and immunological complement is known to interact with cell membranes, the complement system was examined as the agent which might induce the acrosome reaction in vivo. Bovine follicular fluid induces the acrosome reaction in capacitated rabbit spermatozoa in an in vivo assay and this effect could be blocked by antibody to bovine complement component C3. Addition of exogenous purified human C3 to this system overcame the inhibition and the acrosome reation was induced, but C3 alone without follicular fluid had no effect on sperm acrosomes. Similarly, fertilization of rabbit ova in vitro was prevented by antiserum to rabbit complement component C3 while addition of excess human C3 allowed fertilization to proceed. Use of C1 inactivator in bovine follicular fluid did not reduce its activity, but removal of the component properdin by complexing with zymosan resulted in reduced acrosome reactions. This suggests that the alternate pathway may be involved rather than the classical pathway. The total complement system was demonstrated by erythrocyte hemolysis assay to be present in follicular fluid, but extremely low or absent in rabbit oviduct and uterine fluid. Thus, the data presented in this paper show that complement is present in the appropriate fluid and may be involved in the fertilization process of the rabbit by inducing the acrosome reaction in spermatozoa.