An Immunological Model of Trypanosoma (Tkypanozoon) Brucei Brucei in the Vervet Monkey (Cercopithecus Aethiops)

Abstract

The vervet monkey was susceptible to two stocks and a clone of T. brucei. At a dose of 1 x 10"* inoculated parasites the stocks killed the monkeys either in acute or chronic infection. Also, the cloned parasite showed a dose-dependent response. 0 Monkeys died during acute infection when inoculated with 1 x 10 cloned trypanosomes. However, those inoculated with 5 x 10 or 1 x 10 3 parasites gave variable disease pictures and died either during sub-acute or chronic infection particularly when staphylococcal and streptococcal secondary infections supervened. To effectively interpret the results in a T. brucei-infected monkey it was first necessary to establish the normal parameters for conventional cell-mediated responses in a normal uninfected monkey. In the peripheral blood of the vervet monkey, T (erythrocyte rosettes) and B (Stnlg) formed 70 and of 10% lymphocytes, 6 , respectively. Peripheral blood lymphocytes at 2 x 10 per ml, responded optimally to non-specific mitogens, Con A and PHA but not 14 to LPS when cultured for 72 - 120 hours and pulsed with C-TdR for 16 hours terminally, as also did lymphocytes contained in ten-fold diluted whole blood. Meanwhile responder lymphocytes at 3 x 10 per ml were optimally stimulated in an MLC type reaction with an equal concentration of xenogeneic CLA^ cells when co-cultured for 168 hours and pulsed as above. The stimulation by allogeneic or semi-xenogeneic cells from vervet monkeys or humans was poor. The relationship between infection, treatment and self-cure r ^ c t tn i"rr.Mne responses vas alco studied. After immunization with tetanus toxoid, and 2, 4-dinitrochlorobenzene (DNCB) monkeys were inoculated with 1 x 10^ trypanosomes. Subsequently, their lymphocytes were tested in vitro with Con A, PHA and xenogeneic CIA^ cells (in MLC); their DTH tested in vivo with DNCB and their sera examined for specific (anti-trypanosome and anti-tetanus) and non-specific (heterophile) antibodies. Initially total IgM and heterophile antibodies were highly elevated while the IgG levels were only slightly raised. Meanwhile only the primary IgM (but not IgG) anti-tetanus antibodies and the MLR were enhanced. Subsequently, in vitro Con A, PHA and MLC lymphocyte responses, DNCB skin reactivity and both IgM and IgG anti-tetanus antibodies were depressed through the course of infections. Similarly, monkeys that survived the infection without treatment also showed depressed cell-mediated immune parameters. Berenil treatment restored quickly, within 3 days the anti-tetanus antibody responses to normal. However, in vitro Con A, PHA and MLC as well as in vivo skin responses reverted to normal slowly after more than 30 days in the previously T. brucej-infected monkeys. Berenil had no effect on the immune responses of uninfected control monkeys. Monkeys inoculated with 1 x 10 trypanosomes died of acute infection within 2 - 3 weeks while showing depressed lymphocyte responses to Con A and PHA. In the monkeys infected with 1 x 10^ trypanosomes self-cure occurred and cleared the parasitaemia in the blood. However, self-cure did not abrogate the immunodepression as evidenced by persistently depressed stimulation indices from Con A-and PHA-stimulated lymphocytes during the period of absence of detectable parasitaemia and 155 days after the start of the experiment. This suggested that the trypanosomes were perhaps still present in some occluded sites e.g. CNS or tissues, from where they probably exerted their persistent immunodepressive effect. During infection leucocytes and their sub-sets changed numerically and proportionally. In monkeys inoculated with 8 4 1 x 10 or 1 x 10 trypanosomes leucopenia occurred accompanied by lymphocytopenia, neutrocytosis and monocytosis. The lymphocytopenia, was associated with low absolute erythrocyte rosette (T) and surface membrane immunoglobulin-bearing (B) cells and initially reduced but subsequently normal absolute "null" cell counts. The proportions of lymphocytes decreased while those of neutrophils and monocytes increased, respectively. Normocytic and hypochromic anaemia was the most prominent feature of T. brucei infection in the vervets. It persisted in animals that died of the disease but was abrogated by Berenil treatment and self-cure both of which restored to normal the total counts and subpopulations of leucocytes in the chronically infected animals.