Elissa K. Deenick

Macrophages in the skin reside in multiple distinct layers and perform various functions. Here, we show that CD169+ macrophages reside in the hypodermis and comprise the major skin myeloid cell population in the steady state. In a syngeneic melanoma model, CD169+ macrophages encapsulate growing melanomas and directly suppress their growth. CSF1R blockade depleted CD169+ macrophages in tumors and resulted in unrestrained growth. This local containment of tumor growth in the skin was independent of CD169+ subcapsular sinus macrophages in the tumor-draining lymph node and did not require B or T cells. Intravital imaging revealed engulfment and ingestion of live tumor cells by CD169+ macrophages. This phagocytosis did not require the phosphatidylserine receptor MERTK. CD169+ macrophages are also enriched in the hypodermis in skin biopsies from healthy human skin and melanoma. These data identify tissue-resident CD169+ macrophages as a potential cellular target to achieve innate immune containment and reinforce adaptive immune control of tumors.

Nearly 100 individuals have been identified who carry deleterious biallelic germline variants in CARD9 and experience life-threatening, invasive fungal infections caused by Ascomycetes but are otherwise resistant to other infectious agents. CARD9 is an adaptor protein expressed predominantly in myeloid cells, which functions downstream of dectin receptors, pattern recognition receptors for fungal antigens, to activate innate immune responses. The impact of CARD9 deficiency on lymphocytes, however, is less clear. We deciphered the functional consequences and delineated mechanisms of disease in a patient (P1) with a nonsense germline homozygous CARD9 variant (c.673A>T/p.K225*) and invasive Candida disease. P1’s PBMCs expressed truncated CARD9 and showed significantly reduced cytokine production in response to fungal ligands. P1 had reduced frequencies of circulating memory CD4 ⁺ T _H 17-like (CCR6 ⁺ CXCR3 ^– ) cells. In addition, in vitro differentiation of P1’s naive CD4 ⁺ T cells into IL-17A/IL-17F–secreting cells was greatly impaired. Consistent with impaired responses of innate and adaptive immune cells from P1 in vitro, proportions of Candida -specific CD4 ⁺ T cells were strongly and selectively diminished. Our findings suggest that the CARD9 variant identified in P1 is pathogenic, affecting not only CARD9-induced immunity mediated by myeloid cells but also CD4 ⁺ T cell–intrinsic IL-17–dependent immunity and Candida -specific T cell responses.

Activated PI3K delta syndrome 1 (APDS1) is caused by a heterozygous germline gain‐of‐function (GOF) variant in PIK3CD, which encodes the p110δ catalytic subunit of phosphoinositide 3‐kinase (PI3K). APDS1 patients display a broad range of clinical manifestations and perturbations in cellular phenotype. One of the most striking features is the dysregulation of the T‐cell compartment, characterized by an increase in memory T cells, including Tfh cells, and a concomitant decrease in naïve T cells. We have previously shown that many of these changes in T‐cell populations were T‐cell extrinsic and were also induced in WT T cells that developed in the presence of PI3K GOF cells. Here we dissected the drivers of dysregulated T‐cell activation using a mouse model of APDS1. This revealed that PI3K GOF macrophages and DCs made little contribution to the aberrant T‐cell activation. Instead, PI3K GOF T cells were able to drive the loss of WT naïve CD4⁺ T cells, while dysregulated PI3K GOF B cells mediated an increase in Tfh cells. Surprisingly, despite previous reports of increased PI3K signalling driving dysregulated inflammatory Tregs, we saw no evidence for Pik3cd GOF Tregs acquiring an inflammatory phenotype and driving T‐cell activation. These studies provide new insights into the role of PI3K in immune cells and how increased PI3K can drive T‐ and B‐cell dysregulation and contribute to the phenotype of APDS1 patients.

Activated PI3K delta syndrome 1 (APDS1) is caused by a heterozygous germline gain-of-function (GOF) variants in PIK3CD which encodes the p110δ catalytic subunit of phosphoinositide 3-kinase (PI3K). APDS1 patients display a broad range of clinical manifestations and perturbations in cellular phenotype. One of the most striking features is the dysregulation of the T cell compartment characterised by an increase in memory T cells, including Tfh cells, and a concomitant decrease in naïve T cells. We have previously shown that many of these changes in T cell populations were T cell extrinsic and were also induced in WT T cells that developed in the presence of PI3K GOF cells. Here we dissected the drivers of dysregulated T cell activation using a mouse model of APDS1. This revealed PI3K GOF macrophages and DCs made little contribution to the aberrant T cell activation. Instead, the loss of naïve T cells was mostly driven extrinsically by PI3K GOF T cells, while the increase in Tfh cells was mediated by dysregulated PI3K GOF B cells. Surprisingly, despite previous reports of increased PI3K driving dysregulated inflammatory Tregs, we saw no evidence for Pik3cd GOF Tregs acquiring an inflammatory phenotype and driving T cell activation. These studies provide new insights into the clinical phenotype of patients with APDS1 and new understanding of the role of PI3K in immune cells.

B cells develop from hematopoietic stem cells in the bone marrow. Once generated, they serve multiple roles in immune regulation and host defense. However, their most important function is producing antibodies (Ab) that efficiently clear invading pathogens. This is achieved by generating memory B cells that rapidly respond to subsequent Ag exposure, and plasma cells (PCs) that continually secrete Ab. These B cell subsets maintain humoral immunity and host protection against recurrent infections for extended periods of time. Thus, the generation of antigen (Ag)-specific memory cells and PCs underlies long-lived serological immunity, contributing to the success of most vaccines. Our understanding of immunity is often derived from animal models. However, analysis of individuals with monogenic defects that disrupt immune cell function are unprecedented models to link genotypes to clinical phenotypes, establish mechanisms of disease pathogenesis, and elucidate critical pathways for immune cell development and differentiation. Here, we review fundamental breakthroughs in unraveling the complexities of humoral immunity in humans that have come from the discovery of inborn errors disrupting B cell function.

Aberrant AKT activation occurs in a number of cancers, metabolic syndrome, and immune disorders, making it an important target for the treatment of many diseases. To monitor spatial and temporal AKT activity in a live setting, we generated an Akt-FRET biosensor mouse that allows longitudinal assessment of AKT activity using intravital imaging in conjunction with image stabilization and optical window technology. We demonstrate the sensitivity of the Akt-FRET biosensor mouse using various cancer models and verify its suitability to monitor response to drug targeting in spheroid and organotypic models. We also show that the dynamics of AKT activation can be monitored in real time in diverse tissues, including in individual islets of the pancreas, in the brown and white adipose tissue, and in the skeletal muscle. Thus, the Akt-FRET biosensor mouse provides an important tool to study AKT dynamics in live tissue contexts and has broad preclinical applications.

Heterozygous loss-of-function (LOF) mutations in PIK3R1 (encoding phosphatidylinositol 3-kinase [PI3K] regulatory subunits) cause activated PI3Kδ syndrome 2 (APDS2), which has a similar clinical profile to APDS1, caused by heterozygous gain-of-function (GOF) mutations in PIK3CD (encoding the PI3K p110δ catalytic subunit). While several studies have established how PIK3CD GOF leads to immune dysregulation, less is known about how PIK3R1 LOF mutations alter cellular function. By studying a novel CRISPR/Cas9 mouse model and patients’ immune cells, we determined how PIK3R1 LOF alters cellular function. We observed some overlap in cellular defects in APDS1 and APDS2, including decreased intrinsic B cell class switching and defective Tfh cell function. However, we also identified unique APDS2 phenotypes including defective expansion and affinity maturation of Pik3r1 LOF B cells following immunization, and decreased survival of Pik3r1 LOF pups. Further, we observed clear differences in the way Pik3r1 LOF and Pik3cd GOF altered signaling. Together these results demonstrate crucial differences between these two genetic etiologies.

Phosphatidylinositol-3-kinases (PI3K) control many aspects of cellular activation and differentiation and play an important role in B cells biology. Three different classes of PI3K have been described, all of which are expressed in B cells. However, it is the class IA PI3Ks, and the p110δ catalytic subunit in particular, which seem to play the most critical role in B cells. Here we discuss the important role that class IA PI3K plays in B cell development, activation and differentiation, as well as examine what is known about the other classes of PI3Ks in B cells. Keywords B cells PI3K Humoral immunity B cell development Antibody production

Mevalonate kinase deficiency (MKD) is characterized by recurrent fevers and flares of systemic inflammation, caused by biallelic loss-of-function mutations in MVK. The underlying disease mechanisms and triggers of inflammatory flares are poorly understood because of the lack of in vivo models. We describe genetically modified mice bearing the hypomorphic mutation p.Val377Ile (the commonest variant in patients with MKD) and amorphic, frameshift mutations in Mvk. Compound heterozygous mice recapitulated the characteristic biochemical phenotype of MKD, with increased plasma mevalonic acid and clear buildup of unprenylated GTPases in PBMCs, splenocytes, and bone marrow. The inflammatory response to LPS was enhanced in compound heterozygous mice and treatment with the NLRP3 inflammasome inhibitor MCC950 prevented the elevation of circulating IL-1β, thus identifying a potential inflammasome target for future therapeutic approaches. Furthermore, lines of mice with a range of deficiencies in mevalonate kinase and abnormal prenylation mirrored the genotype-phenotype relationship in human MKD. Importantly, these mice allowed the determination of a threshold level of residual enzyme activity, below which protein prenylation is impaired. Elevated temperature dramatically but reversibly exacerbated the deficit in the mevalonate pathway and the defective prenylation in vitro and in vivo, highlighting increased body temperature as a likely trigger of inflammatory flares.