Characterization of human immunoglobulin (Ig) isotype and IgG subclass response to Bartonella henselae infection

Serologic parameters of cat scratch disease (CSD) were evaluated by Western blot analysis. Sera from patients with serologically confirmed CSD antigen were screened for immunoglobulin (Ig) isotype-specific as well as IgG subclass-specific reactivity against Bartonella henselae whole-cell antigen. Bartonella-negative control sera were used to determine baseline antibody activity. Heterogeneous B. henselae-specific IgG reactivity with numerous protein bands, ranging from >150 to <17 kDa, was observed. Though individual banding patterns were variable, one approximately 83-kDa B. henselae protein (Bh83) was immunoreactive with all CSD sera tested, suggesting it is a conserved antigen during infection. Bh83 was not recognized by reference human antisera against Rickettsia rickettsii, Chlamydia group positive, Treponema pallidum, Orientia tsutsugamushi, Fransciscella tularensis, Ehrlichia chaffeensis, Mycoplasma pneumoniae, and Escherichia coli, although other cross-reactive proteins were evident. Significantly, CSD sera failed to recognize the 83-kDa protein when tested against Bartonella quintana antigen, though sera from B. quintana-infected patients did react to Bh83. This cross-reactivity suggests epitope conservation during infection with B. henselae or B. quintana. Western blot analysis further revealed similar banding patterns when B. henselae was reacted against the Ig isotypes IgG and IgG1 and both secretory and alpha chains of IgA. Neither IgM nor IgE reacted significantly to Bartonella antigen by our Western blot analysis. Dissection of the antibody response at the IgG subclass level indicated that prominent antigen recognition was limited to IgG1. These observations provide insight into induced immunity during CSD and provide evidence for conserved epitope expression during infection with B. henselae or B. quintana.