723. IgA1 Protease (IgA1P) Inhibits IgA-Mediated Killing of S. pneumoniae (Spn)
Session: Poster Session: Immunology/Pathogenesis
Saturday, 11 October 2003: 12:00 AM
Room: Exhibit Hall A
IgA1 proteases (IgA1P) may promote the virulence of invasive mucosal pathogens by cleaving Fc from bound IgA, thereby inhibiting phagocyte (PMN) recognition and killing. We generated human monoclonal antibodies (MAb) of the IgA1and IgA2 subclasses that were reactive with type 2 pneumococcal capsular polysaccharide (PPS2) by fusion of K6H6/B5 mouse-human heteromyeloma cells with purified B cells from healthy adults recently immunized with 23-valent pneumococcal polysaccharide vaccine. Preincubation of a PPS2-specific human IgA1 MAb with recombinant IgA1P (from H. influenzae Rd) removed the Fc region and inhibited IgA binding and IgA-dependent killing of Spn in the presence of complement and neutrophils. In mice, the IgA2 MAb, which is not susceptible to IgA1P, provided 60% protection against intranasal challenge of wild type Spn type 2 compared with placebo, whereas the IgA1 MAb provided no protection. Sera from mice receiving purified MAb IgA1 supported killing of Spn type 2 bacteria that produce IgA1P (IgA1P+) only when IgA1 levels were very high (150 µg/mL) 4 hrs after inoculation, but not once levels declined. In contrast, sera from mice receiving MAb IgA2 supported killing of the same IgA1+ Spn at 96 hrs, even at antibody levels as low as 20 ng/mL. Sera containing either IgA1 or IgA2 MAb'sefficiently mediated high levels of killing of an isogenic IgA1P-deficient mutant at all times. The isogenic IgA1P-deficient strain was generated by insertional mutagenesis in the iga gene, as previously described. Finally, passive immunization of mice with the IgA1 MAb (2 µg) provided 60% protection against infection-related death from the IgA1P-deficient mutant but no protection against the isogenic IgA1P+ isolate. In summary, IgA1P protects Spn from killing by specific-IgA1 MAb both in vitro and in vivo. The inhibition can be overwhelmed with very high doses of IgA1. These data suggest that IgA1P may enhance the virulence of the organism by defending against otherwise protective host antibody. . These data direct attention to the role of IgA1P both as a determinant of successful colonization, and as a potential vaccine candidate.
J Weiser, None1, J Rubins, None1, C Fasching, None1, Edward Janoff, MD2, A Plaut, None1 and  E.N. Janoff, None; J.B. Rubins , None; C. Fasching , None; A. Plaut , None; J.N. Weiser , None., (1)Tufts-New England Medical Center; Univ. of Pennsylvania, Minneapolis, MN, (2)VAMC, Minneapolis, MN


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