A secreted pore‐forming protein modulates cellular endolysosomes to augment antigen presentation

Abstract

Bacterial pore‐forming toxin aerolysin‐like proteins are widely distributed in animals and plants. Emerging evidence supports their roles in host innate immunity, but their direct actions in adaptive immunity remain elusive. In this study, we found that βγ‐CAT, an aerolysin‐like protein and trefoil factor complex identified in the frog Bombina maxima, modulated several steps of endocytic pathways during dendritic cell antigen presentation. The protein augmented the antigen uptake of dendritic cells and actively neutralized the acidification of cellular endocytic organelles to favor antigen presentation. In addition, the release of functional exosome‐like extracellular vesicles was largely enhanced in the presence of βγ‐CAT. The cellular action of βγ‐CAT increased the number of major histocompatibility complex (MHC) I‐ovalbumin and MHC II molecules on dendritic cell surfaces and the released exosome‐like extracellular vesicles. An enhanced antigen presentation capacity of dendritic cell for priming of naive T cells was detected in the presence of βγ‐CAT. Collectively, these effects led to strong cytotoxic T lymphocyte responses and antigen‐specific antibody responses. Our findings provide evidence that a vertebrate‐secreted pore‐forming protein can augment antigen presentation by directly modulating cellular endocytic and exocytic pathways, leading to robust activation of adaptive immunity.

A pore-forming protein drives macropinocytosis to facilitate toad water maintaining

“…Previous studies using RT-PCR likewise indicated the expression of βγ-CAT in various B. maxima tissues (23,24). Interestingly, in contrast to its potent cytotoxicity to various mammalian cells (19,25), βγ-CAT only shows toxic effects in toad cells in a dosage up to 2 μM (Fig. S2B), which is much higher than βγ-CAT physiological concentrations observed (23).…”

“…Macropinocytosis, also referred to as cell drinking, is a form of endocytosis that mediates the non-selective uptake of extracellular fluid and solutes (6,12). In a murine DC model, preliminary data showed that βγ-CAT increases the internalization of ovalbumin (antigen), presumed to be mediated by enhanced macropinocytosis (25).…”

“…Water acquisition can be rapidly realized through AQPs and/or macropinocytosis (12,27,28). Previously, in a murine dendritic cell (DC) model, preliminary data suggested that βγ-CAT might enhance pinocytosis (25). In the present study, we carefully studied the ability of this protein to stimulate and participate in macropinocytosis in various types of B. maxima cells.…”

“…This secretory PFP complex βγ-CAT targets gangliosides and sulfatides in cell membranes in a double-receptor binding model (22). Then the BmALP1 subunit is endocytosed and this PFP oligomerizes to form channels on endolysosomes, modulating the contents and biochemical properties of these intracellular organelles (22)(23)(24)(25). Thus, βγ-CAT is a secretory endolysosome channel (SELC) protein, representing a hitherto unknown SELC pathway (17).…”

“…Depending on cell contexts and surroundings, the cellular effects of SELC protein βγ-CAT have been proposed to facilitate the toad in the sense and uptake of environmental materials (like nutrients and antigens) and vesicular transport, while maintaining mucosal barrier function and fulfilling immune defense (17). Accordingly, the roles of βγ-CAT in immune defense have been first documented (21,(23)(24)(25)(26).…”