Immunogenicity of experimental trachoma vaccines in baboons: I. Experimental methods, and preliminary tests with vaccines prepared in chick embryos and in HeLa cells

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Abstract
Parallel titrations of a strain of trachoma (MRC–221) and one of inclusion conjunctivitis (MRC–4) in the baboon conjunctiva and in chick embryos suggest that ten to twenty 50% egg infective doses are equivalent to one 50% baboon infective dose; but that at least 1000 egg infective doses are needed to induce moderate or severe infections in all of a given number of baboons.For vaccine experiments in baboons, a system of scoring physical signs and presence of inclusion bodies was devised; the significance of differences in vaccinated and control animals in their response to conjunctival challenge was determined by analysis of variance. An aqueous suspension of live MRC–4 grown in the yolk sac was given as two subcutaneous doses and one intravenous dose at weekly intervals, and protected all of six baboons challenged with the homologous strain; three similarly spaced subcutaneous doses were less effective. The immunity induced by this vaccine waned considerably during the ensuing 15 months. Vaccine prepared from a live ‘fast-killing’ variant of MRC–4 grown in HeLa cells was less effective than MRC–4 itself in protecting baboons against infection with the parent strain.Although both yolk sac and HeLa cell vaccines induced the formation of antibody fixing complement with trachoma group antigen, the serum titres in individual animals at the time of challenge were unrelated to the degree of protection; during a 15 month observation period there were pronounced falls in the titres of antibody induced either by vaccination or by challenge with egg-grown TRIC agent.We wish to thank Dr I. Sutherland (M.R.C. Statistical Research Unit) for his helpful advice during the early stages of this work. We are also greatly indebted to Mr P. Avis (Pfizer Ltd.) for his advice and for undertaking the statistical computations; and to Miss Anne Smith and Miss Pay Storey (M.R.C. Trachoma Research Unit) for doing the complement fixation tests.