Description:
<jats:p>The protozoan parasite <jats:italic>Leishmania</jats:italic> undergoes a morphological and biochemical transformation from the promastigote to the amastigote form during its life cycle, which is reflected in the expression of stage‐specific proteins. One of these proteins shows homology to a superfamily of reductase proteins. We have cloned the reductase gene from <jats:italic>L. donovani</jats:italic> and have shown that it differs in only one nucleotide from the <jats:italic>L. major</jats:italic> homologue, resulting in one amino acid change. A cytosine (C) to guanine (G) transposition in the coding sequence leads to a nonconserved substitution of asparagine (N) for lysine (K). Only 2 of 22 plasma samples from patients with visceral leishmaniasis were found to have detectable anti‐reductase antibodies and peripheral blood mononuclear cells (PBMC) from one of three individuals previously infected with visceral leishmaniasis proliferated in the presence of recombinant reductase protein. Interestingly, 6 of 10 PBMC isolated from Danish controls proliferated in the presence of the reductase protein. Intracellular IFNγ was found in a significant percentage of cells in all the tested PBMC cultures from Danes, whereas IL4 was only found in a small proportion of cells, or not at all. The results indicate the presence of cross‐reacting CD45R0 memory T‐cells in individuals not exposed to <jats:italic>Leishmania.</jats:italic> Several previous studies have shown that T‐cells from nonexposed individuals often respond to crude <jats:italic>Leishmania</jats:italic> antigen preparations. The present study suggests that this reactivity is partly caused by T‐cells recognising <jats:italic>L. donovani</jats:italic> reductase.</jats:p>