Experimental infection and protection against ... - TI Pharma

162 Chapter 8
inoculated the thawed salivary glands from 2.5 to 15 mosquitoes intravenously
and achieved Pf parasitemia in 13 of 14 volunteers. The only volunteer who did
not develop Pf parasitemia received salivary glands from 2.5 PfSPZ-infected
mosquitoes, the lowest dose. Because the preparations were likely highly
contaminated with bacteria they administered penicillin concomitantly with
PfSPZ-infected salivary gland preparations.
Since 1986 CHMIs have been performed by exposing volunteers to bites of
laboratory-reared mosquitoes infected by feeding on Pf gametocyte-infected
erythrocytes grown in culture [11]. When volunteers are challenged by bites of
five PfSPZ-infected mosquitoes, essentially all develop Pf parasitemia [4, 11, 29,
31]. When numbers are reduced to one or two mosquitoes success rates drop to
50% [30, 32]. Recently, 5 of 6 volunteers became infected when bitten by one
aseptic, PfSPZ-infected mosquito produced in compliance with regulatory
standards [33]. ID inoculation of 2,500 cryopreserved PfSPZ Challenge in the
current study was at least as effective as the bites of 1-2 infected mosquitoes in
infecting volunteers.
The capacity to infect volunteers with PfSPZ Challenge is a function of the
infectiousness of the cryopreserved PfSPZ and the efficiency of administration.
We use in vitro assays of potency and viability to estimate infectiousness of fresh
vs. cryopreserved SPZ. These in vitro assays predicted a maximum difference of
25-30% (Table S1). Rodent model in vivo data, however, suggested that the in
vitro assays underestimate the reduction in infectivity associated with SPZ
cryopreservation (Table S2).
Increasing the dose of PfSPZ Challenge from 2,500 PfSPZ to 25,000 PfSPZ
administered ID neither increased the percentage of infected volunteers nor
reduced time until blood stage parasites were detectable. Parasite density at
diagnosis was lower in the 10,000 PfSPZ group compared to both the lower and
the higher dose groups. This is probably a reflection of natural variability
between the volunteers. Apparently, increasing the dose did not result in higher
numbers of PfSPZ developing in hepatocytes, and invading erythrocytes. A
possible explanation for this lack of dose response may be trapping of PfSPZ at
the inoculation site. Mosquitoes deposit SPZ in the dermis and subcutaneous
tissues in much smaller volumes (< 0.5 μL) than the inocula used in this study (50
μL). This interpretation is consistent with recent studies that showed that
approximately 90% of P. yoelii sporozoites did not reach the liver when
administered ID [34]. Additionally, mosquitoes also inoculate SPZ directly into