Megan A. Cooper

Gain-of-function variants in TLR8 cause X-linked immune dysregulation characterized by cytopenias, bone marrow failure, and lymphoproliferation. We report an 8-year-old male in whom TLR8-GOF manifested as hemophagocytic lymphohistiocytosis, an association not previously described. Treatment with emapalumab was associated with clinical improvement.

Toll-like receptor 8 (TLR8) is an endosomal pattern recognition receptor highly expressed in myeloid cells that senses RNA degradation fragments. Monogenic gain-of-function (GOF) variants in TLR8 cause an inborn error of immunity (IEI) with a spectrum of immune-mediated pathology ranging from severe lymphoproliferation with neutropenia and bone marrow failure to anemia and autoimmunity. Half of identified patients have mosaic disease-causing variants, with variant allele fractions as low as 7% sufficient to drive systemic immune dysregulation. This, along with immunologic profiling of patients, demonstrates a dominant mechanism of disease, likely mediated by inflammatory cytokines. Disease-causing variants in TLR8 are enriched at or near agonist and antagonist binding sites in the encoded protein. As a spectrum of disease and genotypes emerges, identifying sensitivity to agonism and resistance to antagonism may provide insight into molecular mechanisms and therapeutic targets. Here, we review the genetics and clinical features of 19 reported cases of TLR8 GOF diseases, and future areas of investigation for this IEI.

IL-10 receptor deficiency is a life-threatening inborn error of immunity associated with early-onset inflammatory bowel disease. Hematopoietic cell transplantation (HCT) using treosulfan-based conditioning resulted in excellent survival with full donor engraftment. Importantly, anakinra effectively served as a bridge to HCT.

We report severe neutropenia in a 77-year-old female with mosaic TLR8 gain-of-function due to a novel variant. Treatment with ruxolitinib, prednisone, and GM-CSF resulted in clinical improvement. This case expands the phenotypic spectrum beyond pediatric and male cohorts, highlighting mosaicism in late-onset immune dysregulation.

Gain-of-function (GOF) variants in STAT3 cause a complex disorder characterized by early-onset autoimmunity, lymphoproliferation, recurrent infections, and immune dysregulation. In both primary human and mouse models of STAT3 GOF, CD8 ⁺ T cells have been implicated as pathogenic drivers of autoimmunity, though the exact mechanisms remain poorly understood. Here, we found that in patients with STAT3 GOF, CD8 ⁺ T cells exist in an activated state. Functional assessment revealed that naive CD8 ⁺ T cells have an increased capacity for IFN-γ and TNF-α production, with type I and type II IFN transcriptional signatures. Evaluation of immunoregulatory pathways revealed dysregulation of the purinergic signaling axis in CD8 ⁺ T cells: CD39 was increased, whereas downstream purinergic family members, CD73 and the adenosine receptor A _2A R, were downregulated, impairing the potential to produce or sense immunosuppressive adenosine. Evaluation of the impact of precision therapy, in the form of JAK inhibition, at a cellular and functional level revealed partial normalization of CD8 ⁺ T cell dysregulation in patients, including aberrant cytokine production. Our study suggests that a dysregulated purinergic signaling axis plays a key role in CD8 ⁺ T cell dysregulation in STAT3 GOF and may have implications for other rare monogenic immune disorders and common inflammatory disorders.

The requirement for the T-box transcription factors (TF) T-bet and Eomes in innate lymphoid cells (ILCs) beyond their development is not well understood. Here, we generate an inducible, NKp46-specific T-bet knock-out (KO) model and compare it to corresponding Eomes KO and combined T-bet Eomes double KO mice to define T-box TFs requirement in the homeostasis and function of mature NK cell and other NKp46 ⁺ ILC. Inducible T-bet deletion reduces stage IV NK cell numbers in the spleen and tissues, while preserving NK cells in the bone marrow and lymph nodes. Liver ILC1 and small intestine lamina propria NKp46 ⁺ ILC3 are also lost upon T-bet deletion, indicating the requirement for continuous T-bet expression in these ILC types. Combined T-bet and Eomes KO leads to a rapid loss of NK cells, markedly greater than with individual T-bet or Eomes KO. Direct comparison of inducible T-bet and Eomes KO models reveals that Eomes is critical for host protection against murine cytomegalovirus, whereas T-bet is dispensable, despite loss of ILC1. These findings establish the tissue-specific and non-redundant roles for T-box TFs in NKp46 ⁺ ILCs homeostasis and response to viral infection.

Cutaneous T-cell lymphoma (CTCL) remains a challenging disease due to its significant heterogeneity, therapy resistance, and relentless progression. Multiomics technologies offer the potential to provide uniquely precise views of disease progression and response to therapy. Here, we present a comprehensive multiomics view of CTCL clonal evolution, incorporating exome, whole-genome, epigenome, bulk, single-cell T-cell receptor, and single-cell RNA sequencing of 99 clinically annotated serial skin, peripheral blood, and lymph node samples from 34 patients with CTCL. We leveraged this extensive data set to define the molecular underpinnings of CTCL progression in individual patients at single-cell resolution with the goal of identifying clinically useful biomarkers and therapeutic targets. Our studies identified recurrent progression-associated clonal genomic alterations; we highlight mutation of CCR4, phosphoinositide 3-kinase inhibitor signaling, and programmed cell death protein 1 (PD-1) checkpoint pathways as evasion tactics deployed by malignant T cells. We identified a gain-of-function mutation in STAT3 (D661Y) and demonstrated, using cleavage under targets and release using nuclease (CUT&RUN) and RNA sequencing, that it enhances binding to and transcription of genes in Rho GTPase pathways. With our previous work implicating this pathway in histone deacetylase inhibitor–resistant CTCL, these data provide further support for a previously unrecognized role for Rho GTPase pathway dysregulation in CTCL progression. Recurrent progression-associated mutations were common in the epigenetic modifier EZH2, suggesting that EZH2 inhibition may benefit patients with CTCL. Our findings support an approach in which genomic analysis is widely used for improved disease monitoring, biomarker-informed clinical trial design, and genome-guided therapeutic decision-making. Moreover, these molecular changes present new opportunities for therapeutic targeting in this challenging and incurable cancer.

Toll-like receptor 8 (TLR8) gain-of-function (GOF) somatic variants were recently identified as causing severe neutropenia, lymphoproliferation, and immune dysregulation. We report the expanded clinical and laboratory phenotype and management of 10 patients, including the original cohort of 6 male patients. We identify the first female patient with TLR8 GOF who presented during infancy with pure red cell aplasia due to a germ line TLR8 variant, and 2 new disease-causing somatic variants in male patients. Eight patients had somatic mosaicism with peripheral blood variant allele fractions of 7% to 26%, and age of disease onset of 9 months to 28 years. All patients had neutropenia, most with severe neutropenia refractory to medical therapy. Anemia and thrombocytopenia were common. Bone marrow characteristically demonstrated severe myeloid hypoplasia and activated T-cell infiltrates and/or aggregates. An increased number of large granular lymphocytes (LGLs) was identified in 5 patients. Seven patients underwent allogeneic hematopoietic cell transplant (HCT). High rates of post-HCT cytopenia of unclear etiology and graft-versus-host disease were observed. Five patients are surviving at 1 to 3 years after HCT with full donor myeloid and T-cell chimerism, and resolution of disease phenotype. The 2 patients who presented during childhood and did not undergo HCT ultimately died from disease. In conclusion, TLR8 GOF is an X-linked dominant disorder that should be considered in male and female patients with cytopenia, particularly severe neutropenia, lymphoproliferation with immune dysregulation, increased LGLs, and new to this cohort, red cell aplasia. Disease is refractory to medical management, and curative, allogeneic HCT should be considered early after diagnosis. This trial was registered at www.clinicaltrials.gov as #NCT04339777.

The Journal of Human Immunity (JHI) publishes molecular, cellular, and clinical studies of patients with inborn errors of immunity, or their phenocopies, including autoimmune and somatic disorders. A central tenet in the field is that the current range of genetic immunological disorders is only the tip of the iceberg, given the wide range of conditions and populations, and countless patients not yet studied from this angle. Systematic research into causal monogenic lesions is appropriate and likely to be informative in many infectious, allergic, inflammatory, autoimmune, and malignant disorders. Rare or even private genetic etiologies may be of heuristic value, revealing physiological mechanisms disrupted by other, more common genetic or other causes in other patients. In this context, the journal welcomes all immunological studies in line with this vision, in which molecular, cellular, or clinical abnormalities in individuals are seen as candidate phenotypes, potentially driven by inborn errors of immunity—monogenic or otherwise—worthy of genetic investigation.