HLA polymorphisms shape divergent outcomes of Toxoplasma and Plasmodium infection in Eastern Indian HbE/β-thalassemia cohort.

Genetic disorders such as HbE/β-thalassemia (HBT), though deleterious, may undergo positive selection in regions of high parasitic-endemicity, shaping infection outcomes. By integrating NGS and Sanger sequencing from 61 HBT patients and 50 healthy controls, with imaging-based ex-vivo infection-assays and biochemical analysis, we demonstrate, that individuals carrying the HLA-A*33 allele in an Eastern Indian HBT-cohort show significant protection against Toxoplasma gondii infection compared to A*33-negative counterparts. Importantly, this protective phenotype was independent of transfusion frequency. Infection assays in peripheral blood mononuclear cells (PBMCs) confirmed that while parasite entry was unaffected, intracellular replication was markedly restricted in A*33-positive PBMCs, correlating with enhanced CD8⁺ IFN-γ⁺ responses relative to susceptible HLA genotypes. SPR binding studies using recombinant A*33 and Toxoplasma-derived antigenic-peptide SAG2C, revealed that elevated IFN-γ production is linked higher binding affinity of A*33 with SAG2C, leading to improved antigen presentation. Interestingly, A*33 did not confer protection against Plasmodium falciparum, a closely related apicomplexan parasite sharing common ancestry and intracellular lifestyle with Toxoplasma. Instead, NGS-based HLA profiling in this HBT patient-cohort identified a potential negative association between HLA-C*07 and Plasmodium-infection, validated through infection studies in HLA-null K562 cell lines expressing C*07 or A*33. C*07 exhibited strong binding affinity to the Plasmodium-specific MSP3 peptide but not to SAG2C, indicating pathogen-specific antigen recognition with minimal cross-reactivity. These findings highlight the functional interplay between host HLA diversity and apicomplexan antigen specificity, suggesting that selective immune advantages may contribute to the persistence of otherwise deleterious HBT genotypes in Apicomplexan-endemic populations.