| dc.description.abstract | Visceral leishmaniasis is a severe systemic human disease which is always fatal
if left untreated. The treatment is however expensive, painful and has
unpleasant side effects as repeated courses or a change of drugs is often
required. Treatment failure is also becoming a common problem in endemic
areas such as Kenya. With curative treatment proving unsatisfactory,
effective alternative means of control of infection such as vaccination is
necessary. The vervet monkey has been shown to be susceptible to Leishmania
donovani infection and thus provides a good non-human primate model for use
in vaccine development studies.
The following study was undertaken to determine the nature of humoral and
cellular immune response following vaccination with merthiolate killed
Leishmania donovani promastigotes using neuraminidase and galactose ,
oxidase (NAGO) as adjuvant.
Twelve vervet monkeys were used in this study. The animals were divided into
4 groups of 3 animals each. Each animal received 3 intradermal injections at 2
weeks intervals with one of the following; adjuvant plus promastigotes,
promastigotes alone, adjuvant alone or PBS.
Results from ELISA for the presence of anti-promastigote antibody revealed
that there was production of IgG in the animals vaccinated with adjuvant plus
promastigotes and those vaccinated with promastigotes alone though the IgG
levels were low. The absorbance at 1:125 serum dilution was 0.324 for the
adjuvant plus pro mastigote group, 0.410 for the promastigote group, 0.136 for
the adjuvant group, 0.145 for the saline group and 0.954 for an infected animal
used as a positive control.
Western blot analysis showed that the sera from the animals vaccinated with
adjuvant plus promastigotes and those vaccinated with promastigotes alone,
recognized numerous Leishmania donovani promastigote antigens of
molecular weight 18.5-106 KDa. There was also antibody cross-reactivity with
Leishmania major promastigote antigens. High variability was observed in the
pattern of antigen recognition within and among these two groups.
Peripheral blood lymphocytes (PBL) from all the animal groups showed a good
proliferative response upon stimulation with Con A with a stimulation index
(S.I) ranging from 72-137. In addition PBL from the animals vaccinated with
adjuvant plus promastigotes and those vaccinated with promastigotes alone
also showed a good proliferative response upon stimulation with Leishmania
donovani (S.I 10.62 and 5.62) respectively and Leishmania major
promastigote antigens (S.I 4.87 and 3.87) respectively. The response to the
homologous species (l:eishmania donovani) was notably higher.
There was production of detectable levels of IFN-y (29-31 IU/ml) after in vitro
stimulation ofPB~ with Con A in all the animal groups. There was lower/no
IFN-y production after in vitro stimulation ofPBL with leishmanial antigen.
Low levels of IL-4 and IL-2 were detected after in vitro stimulation of PBL
with either Con A or leishmania! antigen il.J all the animal groups .
There was an increase in levels of circulating CD16+, CD8+ and CD45RO+
cells in animals vaccinated with adjuvant plus antigen or with antigen alone.
There was a higher increase. when the adjuvant was included vaccination .
There wasno significant increase in all these cells in the groups of animals
vaccinated with adjuvant or saline alone.
The animals vaccinated with adjuvant plus promastigotes and those vaccinated
with promastigotes alone gave a positive but weak DTE response. The skin
induration was 8.62mm ± 1.02 and 7.88mm ± 1.00 respectively. The skin
induration for the animals vaccinated with adjuvant alone was 5.5mm ± 0.15.
The animals vaccinated with saline alone gave a DTE response of less than
5mm.
From the above results it was apparent that the adjuvant improved the
immunogenicity of the killed Leishmania donovani pro mastigote vaccine.
However the full extent of the protection could have been realised by
challenging the animals with virulent Leishmania donovani promastigote.
Animal challenge experiments are underway in a separate study and have not
been included in this thesis | en |
