SlpA-driven enhancement of immunogenicity and protection against Trypanosoma cruzi using a non-recombinant Lactobacillus acidophilus platform.

The rapid development of effective vaccines, particularly for diseases lacking prophylactic options, remains a critical global health priority. Among these, Chagas disease, caused by the intracellular parasite Trypanosoma cruzi, still has no approved vaccine despite decades of research. Conventional platforms often face limitations in terms of scalability, safety, or immunogenicity, especially when targeting complex parasitic infections. Although Lactobacillus acidophilus has been explored as a probiotic vector, its use in customizable vaccine platforms remains underdeveloped. Here, we evaluated the potential of SlpA, the major surface-layer (S-layer) protein of L. acidophilus, as a structurally stable and immunostimulatory scaffold for antigen presentation. Specifically, we assessed its performance when fused to Tc52, a well-characterized T. cruzi antigen associated with partial protection. We engineered a biosafe, recombinant-free system where purified N-Tc52/SlpA fusion protein and SlpA-coated probiotic bacteria were tested for protective efficacy. The N-Tc52/SlpA fusion consistently enhanced both the magnitude and quality of antibody responses, inducing balanced IgG2c/IgG1 ratios. Upon challenge, animals receiving N-Tc52/SlpA, either in purified form or displayed on L. acidophilus, exhibited reductions in parasite burden in heart and skeletal muscle tissues. Cellular analyses further revealed that both formats promoted activation of splenic CD4+ and CD8+ T cells and expansion of effector and central memory CD4+ subsets, highlighting SlpA's ability to sustain T cell responses. These findings position SlpA as a dual-purpose macromolecule, functioning as both antigen carrier and immunostimulant, and establish a proof of concept for SlpA-based delivery systems adaptable to other intracellular pathogens, particularly where affordability, safety, and stability are crucial.