IRGM3 Contributes to Immunopathology and Is Required for Differentiation of Antigen-Specific Effector CD8⁺T Cells in Experimental Cerebral Malaria

Abstract

C erebral malaria (CM) is the most severe manifestation of Plasmodium falciparum malaria infection in humans, and it is responsible for more than half a million deaths annually, predominantly in sub-Saharan Africa (1). Gamma interferon (IFN-␥) production by leukocytes is a prominent feature of malarial infection. Typically, this IFN-␥ contributes to parasite clearance; however, it may also drive pathology (2). Clinically, the brain dysfunction that occurs during CM manifests as seizures and coma, with progression to death occurring in the absence of treatment. While a definitive understanding of the pathological events underlying CM remains elusive, considerable evidence supports a role for IFN-␥ (3).Infection of C57BL/6 mice with blood-stage Plasmodium berghei ANKA (PbA) leads to experimental cerebral malaria (ECM), which reproduces many features of human CM (4). IFN-␥, produced either by NK cells or by CD4ϩ T cells prior to end-stage disease, markedly increases the expression of major histocompatibility complex I (MHC-I) molecules, ICAM-1 cell adhesion molecules, and CXCR3 ligands in endothelial cells (3,5). Together, these changes contribute to the recruitment of leukocytes, particularly CD8 ϩ T cells, to the brain microvasculature (3, 6). Current evidence indicates that CD8 ϩ T cell-derived IFN-␥ itself does not contribute to pathology (7). Instead, cross-presentation of malaria antigen on central nervous system (CNS) microvascular endothelial cells and recognition by CD8 ϩ cytotoxic T cells (8) leads to endothelial damage in a granzyme B-and perforin-dependent manner (9, 10). Despite the accumulation of knowledge of the effects of IFN-␥ in Plasmodium infection, its actions are highly pleiotropic; therefore, it is likely that IFN-␥-dependent pathways that influence disease progression are yet to be identified.Among the nearly 2,000 genes that are known to be modulated by IFN-␥ (11), the p47 immunity-related GTPases (IRGs) are critical for protection against a range of intracellular bacteria, protozoa, and viruses in diverse cell types (12, 13). A subset of IRGs (IRGM1-IRGM3 in mice and the constitutively expressed IRGMa-IRGMd, resulting from alternative splicing, in humans) Citation Guo J, McQuillan JA, Yau B, Tullo GS, Long CA, Bertolino P, Roediger B, Weninger W, Taylor GA, Hunt NH, Ball HJ, Mitchell AJ. 2015. IRGM3 contributes to immunopathology and is required for differentiation of antigen-specific effector CD8 ϩ T cells in experimental cerebral malaria.

Irgm proteins attenuate inflammatory disease in mouse models of genital Chlamydia infection

“…Irgm proteins limit immune responses by a variety of different mechanisms in different contexts. Specifically, Irgm3 has been shown to regulate inflammation via regulating antigen presentation ( 55 , 56 ), and Irgm1 and Irgm3 regulate T cell responses in genital C. trachomatis infection ( 37 ). Additionally, certain varieties of T cells mediate inflammatory pathology in genital Chlamydia infection ( 6 ).…”

“…Additionally, Irgm2 has been implicated in controlling noncanonical inflammasome activation upon stimulation with LPS or infection with E. coli ( 51 , 52 ). Irgm3 contributes to the regulation of immune responses via antigen presentation ( 55 , 56 ). These mechanisms have also not been investigated in human systems.…”

Human Cerebral Malaria and Experimental Cerebral Malaria in Mice: Relevance and Applicability

“…This protection of Irgm3-/mice was due to less recruitment of CD8+T cells within the brain and low production of inflammatory cytokines. 11 Further studies on immunopathological changes reported that in ECM, pRBCs can be seen in the brain on three days of infection, tissue changes and edema on five days of infection followed by haemorrhage in different areas of brain at 7 th days of infection. 12 The underlying immunopathological change were shown to affect the neurological functions by compromising memory.…”