Complement protein concentrations and activity in human cervical mucus.

Problem: Complement, a system of over 30 interacting proteins, functions as a critical immune mediator at mucosal surfaces including the intestine, airway, and nasal mucosae, where it orchestrates complement-dependent cytotoxicity (CDC), complement-dependent phagocytosis (CDP), and inflammatory responses. While complement components have been detected at low levels in genital tract fluids, including cervical mucus, the physiologic dynamics of complement in the female reproductive tract remain poorly characterized. Notably, systematic quantification of complement component levels across the menstrual cycle has not been conducted, limiting our understanding of how hormonal fluctuations may influence complement-mediated protection at the vaginal mucosa. Method of study: Ten healthy women of reproductive age were recruited to provide paired samples of cervical mucus (CM) and serum (S) during each phase of the menstrual cycle: follicular, ovulatory, and luteal. Samples were analyzed using bead-based multiplex assays to quantify 13 primary complement components. Results: All 13 complement proteins tested were detectable in CM and S; C3b/iC3b was the predominant complement component in CM followed by C4 and C3. In contrast, C4 was the dominant component in S followed by C1q and C3. There were significantly higher levels of C2 in CM during the follicular phase of the menstrual cycle vs. the ovulatory phase (1.15±0.80 vs 0.24±0.22μg/mL), C4b (0.56±0.35 vs 0.22±0.36 μg/mL), C5a (1.45±1.09 vs 0.38±0.48 μg/mL) In contrast, no differences were found in serum complement levels were during the menstrual cycle. Complement concentrations were on average 308-fold (median= 84) lower in CM than in S, except for C3b/iC3b which was only 5 to 7.5-fold lower in CM, and C2 which was higher in CM at the luteal and follicular phases. Midcycle cervical mucus was capable of inducing complement-mediated hemolysis at about 1/3 the potency of serum. Conclusion: Cervical mucus contains detectable and functional levels of complement proteins. These normative values can provide a foundation for future studies on immune mechanisms in the FRT.