Early immune events following experimental infection of lambs with Mycobacterium avium subspecies paratuberculosis.

Abstract

Mycobacterium avium subspecies paratuberculosis is the causative agent of Johne's disease (paratuberculosis), a chronic granulomatous enteritis of ruminants. Clinical disease, which is characterised by emaciation, is seen only in adult animals although infection invariably occurs in the very young. Cell-mediated immunity (CMI) appears to play a central role in the control of infection [ I , 21. CMI responses are strong in subclinical cases, but weak or absent in the clinical condition [3]. Conversely, circulating antibody levels are low in non-clinical cases, but high in animals with Johne's disease [3]. Immune events early in infection are poorly understood and this study investigated changes in gutassociated and peripheral responses in newborn lambs experimentally infected with M.a.paruruberculosis. Sixteen 2-week-old lambs, divided into pairs, were dosed orally on three alternate days with either 5ml of a PBS suspension of >lo' c.f.u. of a cervine isolate of M.a.paratuhercu/osis or with PBS alone. Pairs of lambs were euthanased weekly for 8 weeks after infection. Lymphocytes were isolated from ileal and jejunal Peyer's patches (IPP and JPP), mesenteric lymph node (MLN) and peripheral blood, and phenotyped by flow cytometric analysis [4]. CMI was assessed by proliferative responsiveness and interferongamma (IFN-Y) production by lymphocytes stimulated with mycobacterial antigens [2]. Total immunoglobulin and specific anti-mycobacterial antibody levels were estimated by ELISA. None of the lambs involved in the study showed any overt clinical changes during the course of the experiment. There was considerable individual variability in the phenotypic distribution of lymphocytes in gut-associated tissues (Table I ) . However, there were clear indications that, in infected lambs, CD2' and CD8' T cell numbers were increased in MLN, IPP and JPP, although statistical significance (pc0.05) was only achieved in the latter. In contrast, B cell (CD45R') numbers tended to be lower in the same tissues, with the decrease being significant (pc0.05) in MLN. TIB cell ratios were significantly ( ~ ~ 0 . 0 5 ) increased in both JPP and MLN. Lymphocyte distribution in peripheral blood was unaffected. Changes in phenotypic distribution in vivo were not associated with changes in mitogenand antigen-induced proliferation and IFN-y production in v i m . Gut-associated anti-mycobacterial IgG antibody titres and plasma total IgG levels were significantly (p<0.05) lower in infected animals than in controls. No differences in total IgA concentrations, either in plasma or gut secretions, were detected. Changes in the distribution of CD2' and CD8'cells reported here (Table I ) support previous evidence, in humans [5] and cattle[6], that the earliest immune responses to mycobacterial infection involve T cell-dependent mechanisms. The low levels of specific and non-specific IgG in the infected animals may have been related to the decreased B cell (CD45R') numbers observed. These findings. and the absence of obvious clinical signs, concur with earlier studies in human and bovine tuberculosis suggesting a reciprocal relationship between cellular and humoral immune responses with regard to subclinical and clinical disease status [7, 81. The presence of relatively higher gut-associated antimycobacterial antibody titres in the uninfected controls is more Table 1. Summary of IymDhatic uhenotvuic changes observed in different gut compartments during exuerimental infection of newborn lambs with M.a.mratubercu/osis