TSSA: from yet another T. cruzi surface antigen to the most appealing candidate for the development of parasite serotyping assays.

BUSCAGLIA, CARLOS A.; BALOUZ, VIRGINIA; ROMER, GUADALUPE; CARMONA, SANTIAGO J.; BRACCO, LEONEL; RICCI, ALEJANDRO D.; BERNÁ, LUISA; ROBELLO, CARLOS; ALTCHEH, JAIME; AGÜERO, FERNÁN

Mendoza

Congreso; XI Congreso de la Sociedad Argentina de Protozoología; 2022

Sociedad Argentina de Protozoología

Trypanosoma cruzi, the protozoan agent of Chagas Disease, displays a highly structured population, with multiple strains that could be grouped into six discrete typing units (DTU). In addition to extensive genetic variability, these DTUs present distinct geographical distribution,specific ecological associations with (at least partially) non-overlapping spectra of competent vectors and/or susceptible mammals, and differences in relevant epidemiological/clinical traitssuch as vector infectivity, vector-to-mammal transmissibility, susceptibility to drugs and parasitemia. In this framework, methods able to assign the T. cruzi infecting strain type directlyin blood samples are expected to have a positive effect on both parasite epidemiologic surveillance and clinical management of Chagasic patients. Serological typing assays that exploitthe presence of strain-specific antibody signatures to polymorphic T. cruzi antigens emerge as an appealing approach to address this issue. TSSA (Trypomastigote small surface antigen) was originally identified as yet another parasite antigen, this one encoded by an apparently singlecopy, mucin-like gene expressed by bloodstream trypomastigote forms. Sequencing of TSSA alleles from different strains, however, revealed several polymorphisms that defined 4 main protein variants, each one corresponding to an ancestral DTU (TcI to TcIV). Interestingly, TSSA variants were shown to display major antigenic differences, hence pointing to this molecule as a promising T. cruzi serotyping candidate. Indeed, epidemiological and clinical surveys conducted so far have shown that, despite certain aspects that need to be improved, TSSA is able to provide robust, sensitive, low-cost, and point-of-sampling diagnosis, with near DTU-level resolution. The advent of high-quality parasite genomes showed a larger-than-expected complexity of TSSA sequences. In addition to variations in TSSA gene dosage among T. cruzi strains, our recent genome mining exercises revealed the presence of TSSA pseudogenes and two TSSA hemizygous loci in hybrid DTUs (TcV and TcVI). Most relevant, they allowed the identification of several novel TSSA variants and also one TSSA sequence from the phylogenetically related bat parasite T. cruzi marinkellei. Despite its evident genetic drift, this molecule shared quite similar structural features with T. cruzi TSSAs, thus raising doubts about its evolutionary origin and distribution and, most importantly, warranting further investigations on the diagnostic impact of such atypical variants. Overall, our data shed new light into TSSA evolution, diversity and antigenic landscape, and contribute to improve the design, resolution and specificity of Chagas Disease diagnostic applications, and particularly of T. cruzi serotyping strategies.