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Analysis of human C4A and C4B binding to an immune complex in serum

B D Reilly1

  • 1Department of Health Sciences, University of Wisconsin-Milwaukee, Milwaukee, WI, USA.

Clinical and Experimental Immunology
|July 15, 1999
PubMed

Insights

In serum, complement C4A and C4B bind similarly to immune complexes, unlike isolated proteins. Unexpectedly, C4A binds a 38-kD protein, and higher serum concentrations reduce covalent binding, especially for C4B.

Area of Science:

  • Immunology
  • Biochemistry

Background:

  • Previous studies show isolated C4A binds immune complexes more than C4B.
  • In vivo binding of C4 isoforms to immune complexes remains understudied.

Purpose of the Study:

  • To investigate the in vivo binding of C4A and C4B complement isoforms to immune complexes in serum.
  • To compare in vivo binding with in vitro findings using isolated proteins.

Main Methods:

  • Studied binding of C4A and C4B to bovine serum albumin (BSA) anti-BSA immune complexes in serum.
  • Assessed both covalent and non-covalent binding at varying serum concentrations.

Main Results:

  • In serum, similar amounts of C4A and C4B bound to immune complexes, contrasting with isolated protein studies.
  • C4A specifically bound to an unidentified 38-kD serum protein, while C4B did not.
  • Increased serum concentration decreased covalent binding, with C4B being more affected than C4A.

Conclusions:

  • In vivo complement C4 isoform binding to immune complexes differs from in vitro observations with isolated proteins.
  • The binding of C4A to a 38-kD protein and the serum concentration-dependent decrease in covalent binding warrant further investigation for biological significance.

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