Aspects of humoral and cell-mediated immune responses in calves experimentally infected with Mycobacterium bovis strains

Calves (5; CI to C5) were infected intravenously with 1mg (wet weight) of f reshly grown cultures of M. bovis strain AN5 and a field strain of, M. bovi s (3/87) in 5 mt normal saline solution (NSS). One calf (C6) was administer ed intravenously 5 ml of NSS. Calves C3 and C6 served as control for the tu berculin test since calf C3 (infected) was not tuberculin tested. The calve s C1, C2, C4, C5 and C6 were tuberculin tested with 0.1 ml (2000TU) of PPD of M. bovis AN5 origin at -14 day, 31 day, 76 day and 112 day. Sera from th ese calves and calf C3 were collected at -14 day, -7day and 0 day prior to infection and at weekly intervals from 7 day onwards up to 119 day post ino culation. The sera were assayed for the level of (IgG) antibodies by ELISA using 3 antigens,viz, PPD, heat-ki lied sonicated and M. bovis AN5 species- specific phenolic glycolipid (PGL). The study showed a rise in the IgG leve l at 7 day which persisted up to 14 day, again rose at 42 day to 76 day and then after 112 day post inoculation in calves C1, C2, C4 and C5 when assay ed by ELISA with above PPD and heat killed sonicated antigens. The early ri se in IgG could not be detected by PGL antigen, however, the IgG Could be d etected in these calves from 56 or 63 days which reached plateau after 77 o r 91 days, The antibody level in calf C3 (not tuberculin tested) rose from 105 day post inoculation, whereas, no rise in IgG level could be seen in co ntrol calf C6 (uninfected). Titration of sera from calves by ELISA, with 3 antigens showed that a minimum of 320-640 titre existed and no diagnostic t itre could be attributed. On -14 day no appreciable increase in skin thickn ess was seen in all calves. On 31 day, PPD skin response in C1, C2, C4 and C5 was positive (> 5 min). The skin response on subsequent tuberculin testi ng at 76 day and 122 day was not elevated. The results of cell-mediated and humoral immune responses, suggested a heterogenous humoral response in cal ves and an inverse relationship between cell-mediated and humoral response.