The interactions of polymorphonuclear leukocytes, spermatozoa, seminal plasma, and serum have been examined in an in vitro leukocyte chemotaxis system. Bovine or human spermatozoa have no neutrophil chemotactic activity alone, but generate potent activity when incubated with serum. Generation of this activity requires magnesium, but not calcium ions and is blocked by pre-heating the serum. Chemotactic activity is generated normally in serum deficient in immunoglobulins or C4, but is not produced in serum deficient in C3. Thus, spermatozoa activate complement via the alternative pathway. The chemotactically active agent has been identified as the low m.w. C5 cleavage product (C5a) on the basis of heat stability, gel filtration characteristics, and specific inhibition by antiserum to C5. The spermatozoal constituents responsible for complement activation are largely heat stable, resistant to diisopropylfluorophosphate and on gel filtration compise a heterogeneous group of large m.w. materials.

Bovine seminal plasma has no neutrophil chemotactic activity alone; considerable activity, however, is generated by incubating low, but not high concentrations of seminal plasma with serum. The chemotactic agent produced in serum is C5a. Bovine seminal plasma also contains potent heat labile inhibitors of chemotaxis. Two distinct inhibitors differing in m.w. (50 to 100,000 and 15,000) and heat stability have been isolated. The inhibitors appear to act directly on the neutrophil rather than on the chemotactic factor. Human seminal plasma exhibits slight chemotactic activity alone but does not generate significant additional activity on incubation with serum. In contrast to the inhibitory activity of bovine seminal plasma, the human material contains heat-stable, nondialyzable factors which enhance the neutrophil chemotactic response.

These studies document a specific mechanism for the directed migration of polymorphonuclear leukocytes toward spermatozoa. The findings may provide an explanation for the observed in vivo leukocyte accumulation in the female genital tract in association with spermatozoa and seminal plasma. The biologic significance of this phenomenon relates to potential leukocyte-spermatozoa interaction which may influence the likelihood of fertilization.