(A) Shown are relative units Bb-specific serum IgM (stippled line) and IgG (solid line) of mice at indicated times after infection with host-adapted spirochetes, as assessed by ELISA with a pool of recombinant Borrelia-proteins (DbpA, Arp, OspC, and BmpA). Each data point represents the mean of the group, + SD (n = 5). Data are from one experiment. Serum IgG titers are shown for comparison and have been reported previously (5). (B) Shown are relative units Bb-specific serum IgM to Borrelia proteins Arp, DbpA, OspC, BmpA in non-infected “naïve” mice, as well as B6 (WT) and congenic CD40L−/− mice day 120 post infection (inf) as assessed by ELISA. Symbols indicate results from individual mice and horizontal line indicates the mean for the group. (C) Relative units Arp- (top) and DbpA- (bottom) specific serum IgM of mice (n = 4) at indicated times after infection treated or not with ceftriaxone for 30 days starting on day 45.