Visceral Leishmaniasis in Ververt Monkeys: Immunological Responses During Asymptomatic Infections

Abstract

Nine vervet monkeys (Cercopithecus aethiops) were infected intradermally with 8 x 10(7) virulent L. donovani promastigotes. Four animals developed clinical visceral leishmaniasis and died over a period of 18 months. The remaining five animals have remained asymptomatic for a period of 3 years now. Attempts to isolate parasites from spleen and liver through biopsies were fruitless. Immunological responses of these subclinically infected animals were examined. Enzyme-linked immunosorbent assay (ELISA) and western blot analyses demonstrated Leishmania specific antibodies in these animals, but the antibody titres were low. When proliferation of peripheral blood monocytes (PBMC) to Concanavalin A (Con A) of these animals was compared with control 'disease free animals' there were no significant differences in response. However, L. donovani antigen (fixed promastigotes) specific proliferation was demonstrated in the five subclinically infected animals. High and varying levels of interferon gamma (IFN-gamma) were secreted in PBMC cultures from the five vervet monkeys when stimulated with either Con A or L. donovani antigens. In control animals, IFN-gamma was only detected when PBMC were stimulated with Con A. Marked delayed-type hypersensitivity (DTH) responses were demonstrated in the five subclinically infected animals 48 h after injection with formalin fixed promastgotes. It was concluded that the visceral Leishmania disease spectrum due to L. donovani observed in humans could be induced in vervet monkeys and that L. donovani asymptomatic/cryptic infected animals have competent humoral and cellular responses to homologous parasites.

Subcutaneous immunization against Leishmania major - infection in mice: efficacy of formalin-killed promastigotes combined with adjuvants

“…Preparation of soluble Leishmania antigen (SLA): Antigen was prepared following the method described (GICHERU et al, 1995). Promastigotes were washed briefly and sonicated at 20 kHz for four periods of 40 seconds each on ice, separated by intervals of one minute.…”

“…The assay was performed as described (GICHERU et al, 1995). Polystyrene Micro-ELISA plates (Nunc, Copenhagen, Denmark) were coated overnight with 100 µL of Leishmania major soluble antigen at a concentration of 10 µg/mL, diluted in carbonate-bicarbonate buffer (pH 9.6).…”

Vervet Monkeys Vaccinated with KilledLeishmania majorParasites and Interleukin-12 Develop a Type 1 Immune Response but Are Not Protected against Challenge Infection

“…Purified PBL were adjusted to 2 ϫ 10 7 /ml in complete RPMI medium and stimulated with L. major as described previously (10,25).…”

Safety and skin delayed-type hypersensitivity response in vervet monkeys immunized with Leishmania donovani sonicate antigen delivered with adjuvants

“…Peripheral blood mononuclear cells (PBMCs) were isolated from the whole blood by density gradient centrifugation as described (OLOBO et al, 1990). Purified cells were adjusted to 3 x 10 6 /mL in a complete RPMI 1640 medium and stimulated with Leishmania donovani antigen as previously described (GICHERU et al, 1995). Cultures supernatants were collected from triplicate wells after 72 hours of stimulation and the concentration of TNF-α and IFN-γ in the supernatant was determined by enzyme linked immunosorbent assay (ELISA).…”