157 Cryopreservation of Plasmodium falciparum Sporozoites and Sanaria® PfSPZ Vaccine

Abstract

Controlled Human Malaria Infections (CHMI) used as endpoints in clinical trials have demonstrated that 5 infectious mosquito bites are required to assure infection with Plasmodium falciparum (Pf). If each mosquito injects 100 sporozoites (SPZ), one can estimate a dose of 500 PfSPZ are necessary to infect a human being. For the Sanaria® PfSPZ Challenge, comprising cryopreserved whole sporozoites injected by syringe and needle, it has recently been demonstrated that 3200 PfSPZ are necessary to produce 100% infection in human volunteers. Cryopreservation, therefore, is probably causing approximately a 6.4-fold loss of infectivity. In order for cryopreserved PfSPZ to infect in vivo , they must (a) remain intact, (b) translocate from their site of injection to liver hepatocytes, and (c) develop inside hepatocytes to later parasite stages. We have developed several in vitro assays to quantify the loss in efficacy at each step of the PfSPZ infection process, none of which fully reflects the ∼6.4-fold loss. We therefore conclude that the impact of cryopreservation on PfSPZ may be cumulative, manifest at each stage of the infection sequence and that the overall loss of efficacy is the sum of incremental small losses occurring at each stage. Since we see similar results regardless of whether we test infectious PfSPZ or irradiated PfSPZ (which comprise the Sanaria® PfSPZ Vaccine), the impact of cryopreservation on vaccine protective efficacy is thought to be similar. Source of funding: None declared. Conflict of interest: None declared. aruben@sanaria.com