Systemic and mucosal antibody responses to toxin A in patients infected with Clostridium difficile.

The systemic and mucosal humoral response to toxin A, the primary virulence factor of Clostridium difficile, was measured in sera and intestinal secretions from 21 patients with C. difficile diarrhea, 9 asymptomatic C. difficile fecal excretors, and 10 noncolonized control subjects. Toxin A-specific IgG was higher in convalescent sera of the patients with diarrhea (mean +/- SE, 990 +/- 260 ng/mL) than in acute sera (620 +/- 150 ng/mL), in sera from asymptomatic excretors (410 +/- 140 ng/mL), or control subjects (320 +/- 50 ng/mL; P < .05 convalescent vs. control). The pattern of toxin A-specific serum IgA and intestinal secretory IgA levels was similar to that of serum IgG in these groups. Neutralization of toxin A was demonstrated in 5 of 14 convalescent sera but only 1 of 13 acute sera (P = .04). However, the presence of neutralizing activity was independent of the subsequent clinical response. Therefore, most patients convalescent from C. difficile diarrhea demonstrate systemic and mucosal antibodies to toxin A, but these antibodies following natural infection do not appear to alter the clinical course of C. difficile infection.