Induced pinocytosis and endosomal pathways in Amoeba proteus

Phagolysosomal resistance hypothesized to be a danger signal

Antigen presenting cells sometimes require T cell "help" to kill and decompose microbes they capture, especially when those microbes resist effector molecules including nitric oxide and reactive oxygen species. Pathogens are more likely to resist those effectors, shared by the innate and adaptive immune systems, than are commensals. Does such resistance alert the immune system to the danger posed by those pathogens? Several lines of evidence suggest this occurs. Mouse studies showed a surprising exacerbation, not alleviation of experimental autoimmune encephalomyelitis, by suppression of nitric oxide production, but only when the suppression was applied to animals undergoing vaccination with myelin. In contrast, animals receiving T cells activated by vaccination without suppression of nitric oxide benefitted from reduced autoimmune cytotoxicity when nitric oxide production was suppressed after adoptive transfer. Vaccinia and adenovirus suppress nitric oxide production and have been successful vaccine platforms, also consistent with the above phagolysosomal resistance hypothesis. The hypothesis solves a long-standing quandary-how can nitric oxide protect against both infection and autoimmunity, especially autoimmune diseases for which it seems a major effector? The importance of physical linkage between epitopes, first proposed in Bretscher's Two-Step, Two-Signal theory dependent on B cells, is extended to include phagolysosomal resistance in general, plus a corollary proposition that the immune system detects resistance to dissociation of high-affinity pathogenic ligands from host binding sites to make neutralizing antibodies.

Cryptococcus neoformans Escape From Dictyostelium Amoeba by Both WASH-Mediated Constitutive Exocytosis and Vomocytosis

Cryptococcus neoformans is an environmental yeast that can cause opportunistic infections in humans. As infecting animals does not form part of its normal life-cycle, it has been proposed that the virulence traits that allow cryptococci to resist immune cells were selected through interactions with environmental phagocytes such as amoebae. Here, we investigate the interactions between C. neoformans and the social amoeba Dictyostelium discoideum. We show that like macrophages, D. discoideum is unable to kill C. neoformans upon phagocytosis. Despite this, we find that the yeast pass through the amoebae with an apparently normal phagocytic transit and are released alive by constitutive exocytosis after ~80 min. This is the canonical pathway in amoebae, used to dispose of indigestible material after nutrient extraction. Surprisingly however, we show that upon either genetic or pharmacological blockage of constitutive exocytosis, C. neoformans still escape from D. discoideum by a secondary mechanism. We demonstrate that constitutive exocytosis-independent egress is stochastic and actin-independent. This strongly resembles the non-lytic release of cryptococci by vomocytosis from macrophages, which do not perform constitutive exocytosis and normally retain phagocytosed material. Our data indicate that vomocytosis is functionally redundant for escape from amoebae, which thus may not be the primary driver for its evolutionary selection. Nonetheless, we show that vomocytosis of C. neoformans is mechanistically conserved in hosts ranging from amoebae to man, providing new avenues to understand this poorly-understood but important virulence mechanism.