Targeted and restricted complement activation on acrosome-reacted spermatozoa
Rebecca C. Riley-Vargas, … , Susan Lanzendorf, John P. Atkinson
Rebecca C. Riley-Vargas, … , Susan Lanzendorf, John P. Atkinson
Published May 2, 2005
Citation Information: J Clin Invest. 2005;115(5):1241-1249. https://doi.org/10.1172/JCI23213.
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Article Immunology

Targeted and restricted complement activation on acrosome-reacted spermatozoa

Abstract

A specific hypoglycosylated isoform of the complement regulator membrane cofactor protein (MCP; CD46) is expressed on the inner acrosomal membrane (IAM) of spermatozoa. This membrane is exposed after the acrosome reaction, an exocytosis event that occurs upon contact with the zona pellucida. We initiated this investigation to assess MCP’s regulatory function in situ on spermatozoa. Upon exposure of human spermatozoa to autologous serum or follicular fluid, we unexpectedly observed that acrosome-reacted spermatozoa activated the complement cascade efficiently through C3 but not beyond. Using FACS to simultaneously evaluate viability, acrosomal status, and complement deposition, we found that complement activation was initiated by C-reactive protein (CRP) and was C1q, C2, and factor B dependent. This pattern is consistent with engagement of the classical pathway followed by amplification through the alternative pathway. C3b deposition was targeted to the IAM, where it was cleaved to C3bi. Factor H, and not MCP, was the cofactor responsible for C3b cleavage. We propose that this localized deposition of complement fragments aids in the fusion process between the spermatozoa and egg, in a role akin to that of complement in immune adherence. In addition, we speculate that this “targeted and restricted” form of complement activation on host cells is a common strategy to handle modified self.

Authors

Rebecca C. Riley-Vargas, Susan Lanzendorf, John P. Atkinson

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Figure 7

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Western blot of C3 fragments on AR spermatozoa. (A) Diagram of C3 struct...
Western blot of C3 fragments on AR spermatozoa. (A) Diagram of C3 structure. C3 is composed of 2 chains, α and β. Upon activation, C3a is released, leaving the truncated α′ chain of C3b. Factor I cleaves the α′ chain in 2 locations, creating the fragments α1, α2, and C3f (pictured in yellow). The interchain and intrachain disulfide bonds are also shown. (B) Spermatozoa were incubated with calcium ionophore to induce the acrosome reaction, exposed to 10% autologous human serum, lysed, absorbed with Sepharose linked to mAb against C3d, eluted in reducing SDS buffer, and loaded on a 10–20% gel. Lanes 1–3, C3 standards; lane 4, AR spermatozoa incubated with 10% serum; lane 5, spermatozoa were preincubated with the mAb TRA-2-10 and then exposed to 10% serum; lane 6, spermatozoa were preincubated with the mAb GB24, which blocks the complement-regulatory function of MCP; lane 7, serum was preincubated with the mAb MH-10, which blocks the complement-regulatory function of factor H (46).