The mouse antibody response to infection with Cryptococcus neoformans: V_H and V_L usage in polysaccharide binding antibodies

Cryptococcus neoformans is a ubiquitous fungus that can cause serious infections in humans. The fungus has a polysaccharide (C. neoformans capsular polysaccharide; CNPS) capsule that contributes to its pathogenicity and can elicit an antibody response. Nevertheless, only 4 of 60 BALB/c mice chronically infected with C. neoformans had a detectable increase in serum anti-CNPS. The sera of three responder mice contained both IgM and IgG anti-CNPS antibody, and the titers of λ and κ anti-CNPS antibody were approximately equal. Eight IgM and one IgG3 monoclonal antibodies (mAbs) were generated from the spleen of one responder mouse, and one IgA was generated from the spleen of another mouse. Seven of the IgMs, the IgG3, and the IgA mAb had λ light chains and were specific for serotype D CNPS. Molecular analysis confirmed that this was a highly restricted antibody response. All of the D-specific antibodies used V_H441, J_H3, and either V_λ2/J_λ2 or V_λl/J_λl, and all had the same heavy chain CDR3 amino acid sequence, even though there were differences in the nucleotide sequence of the N/D segment. One IgM mAb reacted with both serotype A and D CNPS, and this mAb used different V_H and J_H genetic elements and had κ light chains. All the anti-CNPS mAbs used J proximal V_H gene elements that have previously been shown to bind dextran and other polysaccharides. Sequence and Southern blot analysis indicate that the serotype-D CNPS-specific mAbs arose from only a few precursor B cells.