Elucidating Parasite and Host Cell Factors Enabling Babesia Infection in Sickle Red Cells under Hypoxic/Hyperoxic Conditions.

Sickle RBCs (SS) represent a naturally existing host-cell resistance mechanism to hemoparasite infections. Here, we investigate the basis of this resistance using Babesia divergens grown in sickle and sickle trait cells. We found that oxygenation and its corresponding effect on RBC-sickling, frequency of fetal hemoglobin (HbF+) cells, the cellular redox environment and parasite proliferation dynamics, all play a role in supporting or inhibiting Babesia proliferation. To identify cellular determinants that supported infection, an image-flow-cytometric tool was developed which could identify sickled-cells and constituent hemoglobin. We showed that hypoxic conditions impaired parasite growth in both HbSS and HbAS cells. Further, cell-sickling was alleviated by oxygenation which in turn relieved the inhibition of parasite proliferation in SS cells under hypoxic conditions. Interestingly, our tool identified HbF+-sickle cells as host-cells of choice under both hypoxia and hyperoxia, which was confirmed using cord RBCs which contain high amounts of HbF. In contrast to AA and AS cells, uninfected HbSS cells showed a higher ROS-containing environment, which was further perturbed on infection. We investigated the parasite's response in hostile SS cells and found that it alters its sub-population structure, with 1N dominance under hypoxic conditions yielding to equivalent ratios of all parasite forms at hyperoxic conditions favorable for its growth. Multiple factors, including oxygenation and its impact on cell-shape, HbF positivity, redox status, and parasite pleiotropy allow Babesia propagation in sickle RBCs. Our studies provide a cellular and molecular basis of natural resistance to Babesia which will aid in defining novel therapies against human babesiosis.