Bacterial persistence and immunity in goats vaccinated with a purE deletion mutant or the parental 16M strain of Brucella melitensis

Abstract

To evaluate host responses, young goats were inoculated subcutaneously with a genetic deletion mutant (deltapurE201) of Brucella melitensis (n = 6), its virulent parental strain 16M (n = 6), or saline (n = 6). No clinical evidence of brucellosis was seen in any goat. Serum antibody titers peaked at postinoculation day (PID) 14. Bacteria in lymph nodes that drained sites of vaccination reached peak numbers of >10(6) CFU/g in both infected groups at PID 7 and progressively declined to PID 84. At necropsy, bacteria were present in mammary lymph nodes or spleen of 33% of goats given virulent 16M but in none of goats given the purE mutant. Lymphadenitis, most severe in goats given 16M, involved depletion of lymphocytes and germinal centers, proliferation of lymphoblasts, and vasculitis. By PID 28, lymph node architecture was restored; there was marked germinal center formation and medullary plasmacytosis. Brucellar antigens, detected with immunoperoxidase techniques, were prominent in capsular granulomas but not in lymph node cortices. Ultrastructurally, bacteria were found in macrophages (>97%) and small lymphocytes (<3%) but not in large lymphocytes. Bacteria were intact in small lymphocytes but in macrophages were in various stages of degradation. The deltapurE phenotype of deltapurE201 was preserved during infection of goat lymph nodes. Unlike Salmonella spp. purE mutants, strain deltapurE201 may be a candidate for efficacy testing; it produced immune responses, was cleared from visceral tissues, and produced less severe pathologic changes than its wild-type parent.