Plant heat shock protein 70 as carrier for immunization against a plant-expressed reporter antigen

Mammalian Heat Shock Proteins (HSP), have potent immune-stimulatory properties due to the natural capability to associate with polypeptides and bind receptors on antigen presenting cells. The present study was aimed to explore whether plant HSP, and in particular HSP70, share similar properties. We wanted in particular to evaluate if HSP70 extracted in association to naturally bound polypeptides from plant tissues expressing a recombinant "reporter" antigen, carry antigen-derived polypeptides and can be used to activate antigen-specific immune responses. This application of HSP70 has been very poorly investigated so far. The analysis started by structurally modeling the plant protein and defining the conditions that ensure maximal expression levels and optimal recovery from plant tissues. Afterwards, HSP70 was purified from Nicotiana benthamiana leaves transiently expressing a heterologous "reporter" protein. The purification was carried out taking care to avoid the release from HSP70 of the polypeptides chaperoned within plant cells. The evaluation of antibody titers in mice sera subsequent to the subcutaneous delivery of the purified HSP70 demonstrated that it is highly effective in priming humoral immune responses specific to the plant expressed "reporter" protein. Overall results indicated that plant-derived HSP70 shares structural and functional properties with the mammalian homologue. This study paves the way to further investigations targeted at determining the properties of HSP70 extracted from plants expressing foreign recombinant antigens as a readily available immunological carrier for the efficient delivery of polypeptides derived from these antigens.