The objective of this project was to investigate effects of isometamidium (ISMM. Samorin®) on
immune responses to Trypanosoma congolense infection in order to understand mechanisms of
prophylaxis. (1) the efficacy of ISMM on T. congolense in vitro and in vivo was examined. (2) effects
on sheep peripheral blood mononuclear cell (PBMC) phenotypes, proliferation, and IFN-y production
were investigated. (3) effects on IL-12 and IFN-y production by mice splenic cells were studied. (4)
effects on sheep PBMC phenotypes following BCG vaccination in sheep were also investigated in
order to establish whether effects of ISMM were specific for trvpanosome antigens.
Three groups consisting of four sheep were used in the trypanosome study. One group was
prophylactically treated with ISMM and one was used as a normal control group. Four and half
months later, all sheep were infected with T. congolense plus a third group that was later treated
with ISMM 14 days post infection. Two groups of three sheep were used in BCG experiments: one
was first treated with ISMM and 14 days after treatment both groups were inoculated with BCG
vaccine. The efficacy of ISMM on T. congolense was studied on cultures in vitro and in sheep and
mice in vivo. IL-12 and IFN-y production by mice splenic cells was investigated at different points
after ISMM treatment
Pre-infection results showed a significant increase in IFN-y production by sheep PBMC 14 to 21 days
after ISMM administration when cultured with live tyrpanosomes. while cultures from the control
group were negative. No significant amounts of IFN-y were detected in all groups during the
infection period. ISMM prophylaxis suppressed polyclonal lymphocyte proliferation in vivo.
Increases in B-cells in the control and treated groups 14 to 21 days after infection were significantly
higher than in the prophy lactic group. A significant decrease in CD4" T-cells was recorded in the
control and treated groups 14 to 21 days post infection, while in the prophylactic group no changes
were observed. CD8~ T-cells increased only after treatment. The ratio of CD4LCD8" T-cells
significantly dropped 21 days after infection in the control and treated groups, while it increased in
the prophylactic group. There were no significant differences in CD5* and yS* T-cell responses.
Trypanosome specific IgG antibodies in serum of the prophy lactic group were significantly higher
than those in the control, while they were absent in the treated group. Following BCG vaccination,
ly mphocy te proliferation in vivo was suppressed in the ISMM treated group. The ratio of CD4LCD8
T-cells was higher in the ISMM group than in the control. Also ISMM. prevented a decrease in the
percentage of CD4~ T cells and suppressed polyclonal CD5" T cells and B cell expansion. Little or no
differences were observed on yd'. CD8" T cells, and PPD skin test. ISMM was trypanostatic in vitro
and T cell suppression decreased the prepatent period in prophylactically treated mice.
In conclusion. ISMM prophylaxis modified cellular and antibody responses to T. congole
infection probably via the IFN-y and 1L-12 feedback mechanisms. This immunomodulation enhanced
prophylaxis and is not specific for trypanosome antigens since similar changes were observed
following BCG vaccination, although the end result of an infection may depend on the type of host
animal and nature of antigen.