A small Heat Shock Protein is Essential for Thermotolerance and Intracellular Survival

10 Leishmania parasites must survive and proliferate in two vastly different environs – the guts of 11 poikilothermic sandflies and the antigen-presenting cells of homeothermic mammals. The change of 12 temperature during transmission from sandflies to mammals is both a key trigger for the 13 progression of their life cycle and for elevated synthesis of heat shock proteins which have been 14 implicated in survival at higher temperatures. While the main heat shock protein families have been 15 studied for their function in the Leishmania life cycle, nothing is known about the roles played by 16 small heat shock proteins. Here, we present first evidence for the pivotal role played by the 17 Leishmania donovani 23-kD heat shock protein which is expressed preferentially in the mammalian 18 stage where it assumes a perinuclear localisation. Loss of HSP23 causes increased sensitivity to 19 chemical stressors, but renders L. donovani incapable of surviving at 37°C. Consequently, HSP23 20 null mutants are non-infectious to primary macrophages in vitro. All phenotypic effect can be 21 abrogated by the introduction of a functional HSP23 transgene into the null mutant, confirming the 22 specificity of the mutant phenotype. Thus, HSP23 expression is a prerequisite for L. donovani 23 survival at mammalian host temperatures and a crucial virulence factor. 24