Serological and Biological Activities of Anti-Haemophilus Influenzae Ribosomal Serum

The antibody content in serum from rabbits immunized with ribosomes from Haemophilus influenzae type b was determined by passive hemagglutination, enzyme-linked immunosorbent assay, and complement fixation. Attempts to use passive hemagglutination to assay anti-ribosomal antibodies were unsuccessful. In the enzyme-linked immunosorbent assay tests, rabbit antiserum was allowed to react with ribosomes that adhered to microtiter plates. The enzyme-linked immunosorbent assay method detected, in two ribosome-immunized rabbits and by 3 days postimmunization, titers which rose to plateaus on days 24 to 31 and declined thereafter. With the complement fixation method, the serum from one immunized rabbit also showed a titer on day 3 and reached a plateau on days 20 to 31. Serum from the other immunized rabbit did not develop a titer until day 11; it reached a lower plateau on days 20 to 24 and then declined on days 27 to 55. Although there were no apparent differences between the two immunized rabbits by the enzyme-linked immunosorbent assay, there were differences between the complement fixation antibodies in these rabbits. Passive protection experiments were performed with these sera. Maximal passive protection was achieved when mice were challenged intraperitoneally with 100 50% lethal doses of H. influenzae and immunized intravenously 1 h later with rabbit serum collected 27 days postimmunization. Rabbit anti-ribosomal sera were evaluated for bactericidal activity. Undiluted immune sera showed bactericidal activity; however, when diluted 1:10, activity was lost. Although bactericidal activities of immune sera were correlated to passive protection activities, it is unlikely that such protection was due to bactericidal antibodies. Immune serum had opsonizing activity since it enhanced phagocytosis of H. influenzae by mouse leukocytes.