EARLY IMMUNOPATHOLOGICAL EVENTS IN EXPERIMENTAL OVINE PARATUBERCULOSIS

An experimental oral infection of neonatal (< 2 weeks old) lambs with a cervine isolate of Mycobacterinm avium subspecies paratuberculosis ( M.a. paratuberculosis), the causal agent of ruminant paratuberculosis (Johne's disease) was used to investigate bacteriological, histopathol ogical and immunological changes during the early (up to 8 weeks) post -infection phase. In vitro culture for mycobacteria was positive in on e faecal and three mesenteric lymph node (MLN) samples from the eight infected lambs. All mycobacterial isolates from MLN were identified as M.a. paratuberculosis by polymerase chain reaction (PCR). Small-to-me dium sized focal granulomata were observed in jejunal (JPP) and ileal Peyer's patches (IPP) from four of the eight infected lambs. Compared with controls, JPP from all infected lambs had significantly (p < 0.05 ) higher proportions of CD8(+) and CD2(+) lymphocytes, and there were significantly (p < 0.05) fewer cells expressing B lymphocyte-associate d markers in IPP and MLN. The T/B cell ratio was significantly (p < 0. 05) increased in both JPP and MLN from infected lambs. The expression of a range of genes for cytokines was examined using specific reverse transcriptase PCR (RT-PCR) amplification of messenger RNA (mRNA) templ ate isolated from MLN, JPP and IPP from both groups of animals. Densit ometric analyses indicated that, in infected animals, MLN expressed si gnificantly (p < 0.05) more mRNA for TNF-alpha JPP had significantly i ncreased (p < 0.05) mRNA for GM-CSF and significantly decreased (p < 0 .05) mRNA for IL-4 and IFN-gamma. Infected lambs had significantly (p < 0.05) decreased titres of both circulating IgG and gut mycobacteria- associated IgG antibody. Infection was not associated with any consist ent changes in lymphocyte reactivity to specific mycobacterial antigen s, IFN-gamma release into supernatants from in vitro intestinal lympho cyte cultures or gut IgA antibody levels. (C) 1998 Elsevier Science B. V. All rights reserved.