Role of complement and the Fc portion of immunoglobulin G in immunity to Venezuelan equine encephalomyelitis virus infection with glycoprotein-specific monoclonal antibodies

We have previously characterized with monoclonal antibodies (MAbs) seven unique epitopes on the two envelope glycoproteins of Venezuelan equine encephalomyelitis (VEE) virus vaccine strain TC-83. The epitopes important in protection from VEE virus infection were determined in passive antibody transfer studies, with virulent VEE (Trinidad donkey) virus as the challenge virus. Selected high-avidity MAbs to the three major protective epitopes (E2(c), E1^b, and E1(d)) were assayed for in vitro complement activity. All three fixed murine complement to high titer. Limited pepsin digestion of the anti-E2(c) in the presence of cysteine resulted in a rapid decrease and complete loss of complement-fixing ability by 2 h, but the majority of mice, except at the lowest dilution of MAb, were protected until the Fc termini were cleaved at 3 h. Anti-E2(c) F(ab')₂ would neutralize VEE (Trinidad donkey) virus more efficiently than either Fab' or Fab; none of the fragments would fix complement or was effective in passive protection. C5-deficient mice and mice depleted of C3 with cobra venom factor were still protected from VEE (Trinidad donkey) virus challenge after passive transfer of either anti-E2(c) or anti-E1^b MAb. The results show that the anti-E2(c) MAb mediates neutralization through bivalent binding at a critical site on the virion and that Fc effector functions, other than complement, are necessary for protection. Although the ability of the anti-E2(c) MAb to fix complement was associated with its ability to protect in vivo, no direct cause-and-effect relationship was found. Since the epitope defined by the anti-E1(d) antibody is found on the cell membrane, but is not expressed on the infectious virion, protection in mice was most likely mediated at the cellular level, possibly by inhibition of the final stages of virion maturation.