Sara E. Hamilton

Cerebral malaria (CM) is a life-threatening complication of Plasmodium infection driven in part by an overwhelming inflammatory response. No clinical therapies are approved that specifically prevent immunopathology during CM. In experimental cerebral malaria (ECM), vascular endothelial cells in the mouse brain cross-present malaria antigen and upregulate adhesion molecules in response to inflammation. This causes CD8 T-cell arrest, activation, and lethal disruption of the blood-brain barrier. We previously reported that treatment with IL-15 cytokine complex (IL-15C) prevents ECM by inducing natural killer (NK) cell secretion of IL-10. To further uncover the mechanisms underlying this immunotherapy, OT-I T cells were transferred into mice, followed by infection with ovalbumin-expressing Plasmodium berghei. Using intravital microscopy, we observed major areas of OT-I arrest within the brains of untreated mice experiencing ECM. IL-15C treatment reduced the number of arresting OT-I T cells, coinciding with an increased mean velocity when compared to untreated mice. To identify the target of NK cell–secreted IL-10, we used conditional knockouts of IL-10Rα and found that protection from ECM occurs through IL-10Rα on endothelial cells and not CD8 T cells. Coinciding with this, IL-15C reduced the expression of adhesion molecules, selectins, and MHC-I on brain endothelial cells. In summary, our studies support the mechanism that IL-15C treatment reduces the number of arrested CD8 T cells along the cerebral vasculature by modulating endothelial cells to prevent ECM. These insights demonstrate the potential for treatments that reduce T-cell arrest to improve outcomes of human CM.

Mice with natural microbial exposures (NMEs) have been proposed to be superior to specific pathogen–free (SPF) laboratory mice for modeling adult human immune and inflammatory profiles. Research using NME mice has focused on the effects of sustained exposure on peripheral blood and lymphoid and nonlymphoid organs. The effect of short-term NME is not known, in particular the impact to bone marrow immune composition. Male and female C57Bl/6J mice were exposed to SPF conditions for 6 wk (age 4 to 10 wk) with or without exposure to dirty bedding from pet store mice for the first week. At sacrifice, substantial shifts in the fecal bacteriome, at the phylum, order, and genus levels were evident in male and female NME mice, confirming induction of a new exposome with NME. However, only female NME mice had pathogens in their feces detectable by polymerase chain reaction (2 of 35). Numerous sex-dependent differences in T cell and myeloid composition in bone marrow and spleens were observed. We report several novel immune composition findings by sex, including higher proportions of Ki67+ regulatory and CD8+ T cells in female bone marrow. We detected 2 differences resulting from NME in the spleens of females, including increased PD-1 in CD44+CD8+ T cells and higher mean Foxp3 expression in regulatory T cells. We conclude that short-term exposure to pet store bedding is insufficient to induce robust changes in immune composition in spleens or bone marrow. Future research with the NME model should focus on sustained exposure and investigate exposure timing.

Memory CD8⁺ T (T_mem) cells are activated into innate-like killers by cytokines, including interleukin-12 (IL-12), IL-15 and IL-18; but mechanisms regulating this phenomenon (termed bystander activation) are unclear. Here we show that basal IL-4 signals antagonize IL-18 sensing and subsequent interferon-γ production during T_mem cell bystander activation. IL-4 treatment can act directly on T_mem cells in a STAT6-dependent manner to limit interferon-γ-mediated control of a bystander bacterial infection. IL-4 does not simply block bystander activation but tunes effector molecule expression. Strain-specific defects in bystander activation of homeostatic T_mem cells partially relates to IL-4 exposure, but these differences are erased in T_mem cells produced by T cell antigen receptor activation, leading to uniform IL-18 receptor expression and capacity for bystander activation/cytotoxicity. Our data demonstrate that bystander activation by inflammatory cytokines is subject to regulation by both IL-4 and prior antigen experience. These findings underscore the importance of the cytokine milieu in dictating bystander-mediated pathogen control.

Mice with normalized microbial exposure (NME) harbor an immune system that more accurately reflects that of humans compared to mice maintained as specific pathogen-free (SPF). An explanation for the observed alterations in the composition of the T cell compartment in NME mice has not been reported. We compared the T cell landscape in NME versus SPF mice at baseline and after acute LCMV infection. Using the immgenT dataset, we found no unique T cell populations in NME, but the landscape shifted towards activated T cells with increased propensity for effector functions and improved pathogen clearance. CD8 ⁺ KLRG1 ⁺ cells (immgenT CD8_cl12) are significantly expanded in NME mice. Their predominance was a result of both increased formation and the conversion of other memory populations to a KLRG1+ phenotype. Thus, NME mice provide insight into a diverse T cell compartment rich with cells previously found to be limited in SPF mice.

P.falciparum infection can trigger high levels of inflammation that lead to fever and sometimes severe disease. People living in malaria-endemic areas gradually develop resistance to symptomatic malaria and control both parasite numbers and the inflammatory response. We previously found that adaptive NK cells correlated with reduced parasite load and protection from symptoms. We also found that murine NK cell production of IL-10 protected mice from experimental cerebral malaria. Human NK cells can also secrete IL-10, but it is unknown what NK cell subsets produce IL-10 or if this is affected by malaria experience. We hypothesized that NK cell immunoregulation may lower inflammation and reduce fever induction. Here, we showed that NK cells from participants with malaria experience make significantly more IL-10 than participants with no malaria experience. We then determined the proportions of NK cells that are cytotoxic and produce IFN-γ and/or IL-10 and identified a signature of adaptive and checkpoint molecules on IL-10–producing NK cells. Lastly, we found that coculture with primary monocytes, Plasmodium -infected RBCs, and antibody induced IL-10 production by NK cells. These data suggest that NK cells may contribute to protection from malaria symptoms via IL-10 production.

Parasitic infections are a major worldwide health burden, yet most studies of CD8 T cell differentiation focus on acute viral and bacterial infections. To understand effector and memory CD8 T cell responses during erythrocytic malaria infection in mice, we utilized transgenic OT-I T cells and compared CD8 T cell responses between infection with OVA-expressing strains of Listeria monocytogenes (Lm) and Plasmodium berghei ANKA (PbA). We find that CD8 T cells expand vigorously during both infections. However, in contrast to Lm infection, PbA infection induces T cells that are heavily biased toward an IL-7Ra-deficient and KLRG1+ short-lived effector cell (SLEC) phenotype at the expense of memory precursor effector cell (MPECs) formation. PbA-induced inflammation, including IFNγ, is partially responsible for this outcome. Following treatment with antimalarial drugs and T cell contraction, PbA-primed memory T cells are rarely found in the blood and peripheral tissues but do maintain a low presence in the spleen and bone marrow. Despite these poor numbers, PbA memory T cells robustly expand upon vaccination or viral infection, control pathogen burden, and form secondary memory pools. Thus, despite PbA enforced SLEC formation and limited memory, effective secondary responses can still proceed.

KLRG1 ⁺ CD8 T cells persist for months after clearance of acute infections and maintain high levels of effector molecules, contributing protective immunity against systemic pathogens. Upon secondary infection, these long-lived effector cells (LLECs) are incapable of forming other circulating KLRG1 ⁻ memory subsets such as central and effector memory T cells. Thus, KLRG1 ⁺ memory T cells are frequently referred to as a terminally differentiated population that is relatively short lived. Here, we show that after viral infection of mice, effector cells derived from LLECs rapidly enter nonlymphoid tissues and reduce pathogen burden but are largely dependent on receiving antigen cues from vascular endothelial cells. Single-cell RNA sequencing reveals that secondary memory cells in nonlymphoid tissues arising from either KLRG1 ⁺ or KLRG1 ⁻ memory precursors develop a similar resident memory transcriptional signature. Thus, although LLECs cannot differentiate into other circulating memory populations, they still retain the flexibility to enter tissues and establish residency.

The melanoma tumor microenvironment is a complex milieu of cancer, inflammatory, and stromal cells. In this context, chemokines play a pivotal role in recruiting inflammatory cells and influence the tumor, exerting both pro‐tumorigenic and anti‐tumoral roles. Interactions between these cells is what ultimately hold together and transform the tumor into an efficient machine. A recent study found that chemokines CCL8, CCL15, and CCL20 were upregulated in melanoma cells when co‐cultured with macrophages and were associated with poor survival rates. CCL8 and CCL15 also stimulated melanoma cell growth, invasion, and metastasis, and were highly expressed in tumors prone to metastasize, suggesting these chemokines are attractive and independent biomarkers. Understanding the intricated interactions within the tumor microenvironment could lead to prognostic biomarkers and to the development of new therapeutic strategies for melanoma. © 2024 The Author(s). The Journal of Pathology published by John Wiley & Sons Ltd on behalf of The Pathological Society of Great Britain and Ireland.

The National Institute of Allergy and Infectious Diseases (NIAID) hosted a two-day virtual workshop on leveraging microbial exposure to improve mouse models of human immune status and disease. The workshop’s objective was to evaluate the current state of knowledge in the field and to identify gaps, challenges and future directions.