Fatal Central Nervous System Lymphocytic Vasculitis after Treatment for Burkitt Lymphoma in a Patient with a SH2D1A Mutation

Very rarely, patients with X-linked lymphoproliferative syndrome type 1 present central nervous system vasculitis. We report a patient carrying a SH2D1A mutation that, after treatment for lymphoma developed fatal central nervous system vasculitis. He lacked signs of ongoing Epstein-Barr virus infection. We propose that impaired T cell homeostasis caused by SAP deficiency facilitates aberrant CD8 T cell activation against vascular antigens promoting clinical manifestations.info:eu-repo/semantics/publishedVersio

Serial Killing of Tumor Cells by Human Natural Killer Cells – Enhancement by Therapeutic Antibodies

BACKGROUND: Natural killer cells are an important component of the innate immune system. Anti-cancer therapies utilizing monoclonal antibodies also rely on the cytotoxicity of NK cells for their effectiveness. Here, we study the dynamics of NK cell cytotoxicity. METHODOLOGY/PRINCIPAL FINDINGS: We observe that IL-2 activated human NK cells can serially hit multiple targets. Using functional assays, we demonstrate that on an average, a single IL-2 activated NK cell can kill four target cells. Data using live video microscopy suggest that an individual NK cell can make serial contacts with multiple targets and majority of contacts lead to lysis of target cells. Serial killing is associated with a loss of Perforin and Granzyme B content. A large majority of NK cells survive serial killing, and IL-2 can replenish their granular stock and restore the diminished cytotoxicity of ‘exhausted’ NK cells. IL-2 and IL-15 are equally effective in enhancing the killing frequency of resting NK cells. Significantly, Rituximab, a therapeutic monoclonal antibody increases the killing frequency of both resting and IL-2 activated NK cells. CONCLUSION/SIGNIFICANCE: Our data suggest that NK cell-based therapies for overcoming tumors rely on their serial killing ability. Therefore, strategies augmenting the killing ability of NK cells can boost the immune system and enhance the effectiveness of monoclonal antibody-based therapies

A biophysical model of cell adhesion mediated by immunoadhesin drugs and antibodies

A promising direction in drug development is to exploit the ability of natural killer cells to kill antibody-labeled target cells. Monoclonal antibodies and drugs designed to elicit this effect typically bind cell-surface epitopes that are overexpressed on target cells but also present on other cells. Thus it is important to understand adhesion of cells by antibodies and similar molecules. We present an equilibrium model of such adhesion, incorporating heterogeneity in target cell epitope density and epitope immobility. We compare with experiments on the adhesion of Jurkat T cells to bilayers containing the relevant natural killer cell receptor, with adhesion mediated by the drug alefacept. We show that a model in which all target cell epitopes are mobile and available is inconsistent with the data, suggesting that more complex mechanisms are at work. We hypothesize that the immobile epitope fraction may change with cell adhesion, and we find that such a model is more consistent with the data. We also quantitatively describe the parameter space in which binding occurs. Our results point toward mechanisms relating epitope immobility to cell adhesion and offer insight into the activity of an important class of drugs.Comment: 13 pages, 5 figure

Cellular expression, trafficking, and function of two isoforms of human ULBP5/RAET1G

Background: The activating immunoreceptor NKG2D is expressed on Natural Killer (NK) cells and subsets of T cells. NKG2D contributes to anti-tumour and anti-viral immune responses in vitro and in vivo. The ligands for NKG2D in humans are diverse proteins of the MIC and ULBP/RAET families that are upregulated on the surface of virally infected cells and tumours. Two splicing variants of ULBP5/RAET1G have been cloned previously, but not extensively characterised. Methodology/Principal Findings: We pursue a number of approaches to characterise the expression, trafficking, and function of the two isoforms of ULBP5/RAET1G. We show that both transcripts are frequently expressed in cell lines derived from epithelial cancers, and in primary breast cancers. The full-length transcript, RAET1G1, is predicted to encode a molecule with transmembrane and cytoplasmic domains that are unique amongst NKG2D ligands. Using specific anti-RAET1G1 antiserum to stain tissue microarrays we show that RAET1G1 expression is highly restricted in normal tissues. RAET1G1 was expressed at a low level in normal gastrointestinal epithelial cells in a similar pattern to MICA. Both RAET1G1 and MICA showed increased expression in the gut of patients with celiac disease. In contrast to healthy tissues the RAET1G1 antiserum stained a wide variety or different primary tumour sections. Both endogenously expressed and transfected RAET1G1 was mainly found inside the cell, with a minority of the protein reaching the cell surface. Conversely the truncated splicing variant of RAET1G2 was shown to encode a soluble molecule that could be secreted from cells. Secreted RAET1G2 was shown to downregulate NKG2D receptor expression on NK cells and hence may represent a novel tumour immune evasion strategy. Conclusions/Significance: We demonstrate that the expression patterns of ULBP5RAET1G are very similar to the well-characterised NKG2D ligand, MICA. However the two isoforms of ULBP5/RAET1G have very different cellular localisations that are likely to reflect unique functionality

Matched sizes of activating and inhibitory receptor/ligand pairs are required for optimal signal integration by human Natural Killer cells

It has been suggested that receptor-ligand complexes segregate or co-localise within immune synapses according to their size, and this is important for receptor signaling. Here, we set out to test the importance of receptor-ligand complex dimensions for immune surveillance of target cells by human Natural Killer (NK) cells. NK cell activation is regulated by integrating signals from activating receptors, such as NKG2D, and inhibitory receptors, such as KIR2DL1. Elongating the NKG2D ligand MICA reduced its ability to trigger NK cell activation. Conversely, elongation of KIR2DL1 ligand HLA-C reduced its ability to inhibit NK cells. Whereas normal-sized HLA-C was most effective at inhibiting activation by normal-length MICA, only elongated HLA-C could inhibit activation by elongated MICA. Moreover, HLA-C and MICA that were matched in size co-localised, whereas HLA-C and MICA that were different in size were segregated. These results demonstrate that receptor-ligand dimensions are important in NK cell recognition, and suggest that optimal integration of activating and inhibitory receptor signals requires the receptor-ligand complexes to have similar dimensions

Dual requirement of cytokine and activation receptor triggering for cytotoxic control of murine cytomegalovirus by NK cells

Natural killer (NK) cells play a critical role in controlling murine cytomegalovirus (MCMV) and can mediate both cytokine production and direct cytotoxicity. The NK cell activation receptor, Ly49H, is responsible for genetic resistance to MCMV in C57BL/6 mice. Recognition of the viral m157 protein by Ly49H is sufficient for effective control of MCMV infection. Additionally, during the host response to infection, distinct immune and non-immune cells elaborate a variety of pleiotropic cytokines which have the potential to impact viral pathogenesis, NK cells, and other immune functions, both directly and indirectly. While the effects of various immune deficiencies have been examined for general antiviral phenotypes, their direct effects on Ly49H-dependent MCMV control are poorly understood. To specifically interrogate Ly49H-dependent functions, herein we employed an in vivo viral competition approach to show Ly49H-dependent MCMV control is specifically mediated through cytotoxicity but not IFNγ production. Whereas m157 induced Ly49H-dependent degranulation, efficient cytotoxicity also required either IL-12 or type I interferon (IFN-I) which acted directly on NK cells to produce granzyme B. These studies demonstrate that both of these distinct NK cell-intrinsic mechanisms are integrated for optimal viral control by NK cells

The CD85j+ NK Cell Subset Potently Controls HIV-1 Replication in Autologous Dendritic Cells

Natural killer (NK) cells and dendritic cells (DC) are thought to play critical roles in the first phases of HIV infection. In this study, we examined changes in the NK cell repertoire and functions occurring in response to early interaction with HIV-infected DC, using an autologous in vitro NK/DC coculture system. We show that NK cell interaction with HIV-1-infected autologous monocyte-derived DC (MDDC) modulates NK receptor expression. In particular, expression of the CD85j receptor on NK cells was strongly down-regulated upon coculture with HIV-1-infected MDDC. We demonstrate that CD85j+ NK cells exert potent control of HIV-1 replication in single-round and productively HIV-1-infected MDDC, whereas CD85j− NK cells induce a modest and transient decrease of HIV-1 replication. HIV-1 suppression in MDCC by CD85j+ NK cells required cell-to-cell contact and did not appear mediated by cytotoxicity or by soluble factors. HIV-1 inhibition was abolished when NK-MDDC interaction through the CD85j receptor was blocked with a recombinant CD85j molecule, whereas inhibition was only slightly counteracted by blocking HLA class I molecules, which are known CD85j ligands. After masking HLA class I molecules with specific antibodies, a fraction of HIV-1 infected MDDC was still strongly stained by a recombinant CD85j protein. These results suggest that CD85j+ NK cell inhibition of HIV-1 replication in MDDC is mainly mediated by CD85j interaction with an unknown ligand (distinct from HLA class I molecules) preferentially expressed on HIV-1-infected MDDC

Expression Analysis of the Ligands for the Natural Killer Cell Receptors NKp30 and NKp44

BACKGROUND: The natural cytotoxicity receptors (NCR) are important to stimulate the activity of Natural Killer (NK) cells against transformed cells. Identification of NCR ligands and their level of expression on normal and neoplastic cells has important implications for the rational design of immunotherapy strategies for cancer. METHODOLOGY/PRINCIPAL FINDINGS: Here we analyze the expression of NKp30 ligand and NKp44 ligand on 30 transformed or non-transformed cell lines of different origin. We find intracellular and surface expression of these two ligands on almost all cell lines tested. Expression of NKp30 and NKp44 ligands was variable and did not correlate with the origin of the cell line. Expression of NKp30 and NKp44 ligand correlated with NKp30 and NKp44-mediated NK cell lysis of tumor cells, respectively. The surface expression of NKp30 ligand and NKp44 ligand was sensitive to trypsin treatment and was reduced in cells arrested in G(2)/M phase. CONCLUSION/SIGNIFICANCE: These data demonstrate the ubiquitous expression of the ligands for NKp30 and NKp44 and give an important insight into the regulation of these ligands

Autoimmunity, hypogammaglobulinemia, lymphoproliferation and mycobacterial disease in patients with dominant activating mutations in STAT3

The signal transducer and activator of transcription (STAT) family of transcription factors orchestrate hematopoietic cell differentiation. Recently, mutations in STAT1, STAT5B, and STAT3 have been linked to development of IPEX-like syndrome. Here, we immunologically characterized three patients with de novo activating mutations in the DNA binding or dimerization domains of STAT3 (p.K392R, p.M394T and p.K658N, respectively). The patients displayed multi-organ autoimmunity, lymphoproliferation, and delayed-onset mycobacterial disease. Immunologically, we noted hypogammaglobulinemia with terminal B cell maturation arrest, dendritic cell deficiency, peripheral eosinopenia, increased double-negative (CD4-CD8-) T cells, and decreased NK, Th17, and regulatory T cell numbers. Notably, the patient harboring the K392R mutation developed T cell LGL leukemia at age 14. Our results broaden the spectrum of phenotypes caused by activating STAT3 mutations, highlight the role of STAT3 in the development and differentiation of multiple immune cell lineages, and strengthen the link between the STAT family of transcription factors and autoimmunity.The signal transducer and activator of transcription (STAT) family of transcription factors orchestrate hematopoietic cell differentiation. Recently, mutations in STAT1, STAT5B, and STAT3 have been linked to development of IPEX-like syndrome. Here, we immunologically characterized three patients with de novo activating mutations in the DNA binding or dimerization domains of STAT3 (p.K392R, p.M394T and p.K658N, respectively). The patients displayed multi-organ autoimmunity, lymphoproliferation, and delayed-onset mycobacterial disease. Immunologically, we noted hypogammaglobulinemia with terminal B cell maturation arrest, dendritic cell deficiency, peripheral eosinopenia, increased double-negative (CD4-CD8-) T cells, and decreased NK, Th17, and regulatory T cell numbers. Notably, the patient harboring the K392R mutation developed T cell LGL leukemia at age 14. Our results broaden the spectrum of phenotypes caused by activating STAT3 mutations, highlight the role of STAT3 in the development and differentiation of multiple immune cell lineages, and strengthen the link between the STAT family of transcription factors and autoimmunity.The signal transducer and activator of transcription (STAT) family of transcription factors orchestrate hematopoietic cell differentiation. Recently, mutations in STAT1, STAT5B, and STAT3 have been linked to development of IPEX-like syndrome. Here, we immunologically characterized three patients with de novo activating mutations in the DNA binding or dimerization domains of STAT3 (p.K392R, p.M394T and p.K658N, respectively). The patients displayed multi-organ autoimmunity, lymphoproliferation, and delayed-onset mycobacterial disease. Immunologically, we noted hypogammaglobulinemia with terminal B cell maturation arrest, dendritic cell deficiency, peripheral eosinopenia, increased double-negative (CD4-CD8-) T cells, and decreased NK, Th17, and regulatory T cell numbers. Notably, the patient harboring the K392R mutation developed T cell LGL leukemia at age 14. Our results broaden the spectrum of phenotypes caused by activating STAT3 mutations, highlight the role of STAT3 in the development and differentiation of multiple immune cell lineages, and strengthen the link between the STAT family of transcription factors and autoimmunity.The signal transducer and activator of transcription (STAT) family of transcription factors orchestrate hematopoietic cell differentiation. Recently, mutations in STAT1, STAT5B, and STAT3 have been linked to development of IPEX-like syndrome. Here, we immunologically characterized three patients with de novo activating mutations in the DNA binding or dimerization domains of STAT3 (p.K392R, p.M394T and p.K658N, respectively). The patients displayed multi-organ autoimmunity, lymphoproliferation, and delayed-onset mycobacterial disease. Immunologically, we noted hypogammaglobulinemia with terminal B cell maturation arrest, dendritic cell deficiency, peripheral eosinopenia, increased double-negative (CD4-CD8-) T cells, and decreased NK, Th17, and regulatory T cell numbers. Notably, the patient harboring the K392R mutation developed T cell LGL leukemia at age 14. Our results broaden the spectrum of phenotypes caused by activating STAT3 mutations, highlight the role of STAT3 in the development and differentiation of multiple immune cell lineages, and strengthen the link between the STAT family of transcription factors and autoimmunity.The signal transducer and activator of transcription (STAT) family of transcription factors orchestrate hematopoietic cell differentiation. Recently, mutations in STAT1, STAT5B, and STAT3 have been linked to development of IPEX-like syndrome. Here, we immunologically characterized three patients with de novo activating mutations in the DNA binding or dimerization domains of STAT3 (p.K392R, p.M394T and p.K658N, respectively). The patients displayed multi-organ autoimmunity, lymphoproliferation, and delayed-onset mycobacterial disease. Immunologically, we noted hypogammaglobulinemia with terminal B cell maturation arrest, dendritic cell deficiency, peripheral eosinopenia, increased double-negative (CD4-CD8-) T cells, and decreased NK, Th17, and regulatory T cell numbers. Notably, the patient harboring the K392R mutation developed T cell LGL leukemia at age 14. Our results broaden the spectrum of phenotypes caused by activating STAT3 mutations, highlight the role of STAT3 in the development and differentiation of multiple immune cell lineages, and strengthen the link between the STAT family of transcription factors and autoimmunity.The signal transducer and activator of transcription (STAT) family of transcription factors orchestrate hematopoietic cell differentiation. Recently, mutations in STAT1, STAT5B, and STAT3 have been linked to development of IPEX-like syndrome. Here, we immunologically characterized three patients with de novo activating mutations in the DNA binding or dimerization domains of STAT3 (p.K392R, p.M394T and p.K658N, respectively). The patients displayed multi-organ autoimmunity, lymphoproliferation, and delayed-onset mycobacterial disease. Immunologically, we noted hypogammaglobulinemia with terminal B cell maturation arrest, dendritic cell deficiency, peripheral eosinopenia, increased double-negative (CD4-CD8-) T cells, and decreased NK, Th17, and regulatory T cell numbers. Notably, the patient harboring the K392R mutation developed T cell LGL leukemia at age 14. Our results broaden the spectrum of phenotypes caused by activating STAT3 mutations, highlight the role of STAT3 in the development and differentiation of multiple immune cell lineages, and strengthen the link between the STAT family of transcription factors and autoimmunity.The signal transducer and activator of transcription (STAT) family of transcription factors orchestrate hematopoietic cell differentiation. Recently, mutations in STAT1, STAT5B, and STAT3 have been linked to development of IPEX-like syndrome. Here, we immunologically characterized three patients with de novo activating mutations in the DNA binding or dimerization domains of STAT3 (p.K392R, p.M394T and p.K658N, respectively). The patients displayed multi-organ autoimmunity, lymphoproliferation, and delayed-onset mycobacterial disease. Immunologically, we noted hypogammaglobulinemia with terminal B cell maturation arrest, dendritic cell deficiency, peripheral eosinopenia, increased double-negative (CD4-CD8-) T cells, and decreased NK, Th17, and regulatory T cell numbers. Notably, the patient harboring the K392R mutation developed T cell LGL leukemia at age 14. Our results broaden the spectrum of phenotypes caused by activating STAT3 mutations, highlight the role of STAT3 in the development and differentiation of multiple immune cell lineages, and strengthen the link between the STAT family of transcription factors and autoimmunity.The signal transducer and activator of transcription (STAT) family of transcription factors orchestrate hematopoietic cell differentiation. Recently, mutations in STAT1, STAT5B, and STAT3 have been linked to development of IPEX-like syndrome. Here, we immunologically characterized three patients with de novo activating mutations in the DNA binding or dimerization domains of STAT3 (p.K392R, p.M394T and p.K658N, respectively). The patients displayed multi-organ autoimmunity, lymphoproliferation, and delayed-onset mycobacterial disease. Immunologically, we noted hypogammaglobulinemia with terminal B cell maturation arrest, dendritic cell deficiency, peripheral eosinopenia, increased double-negative (CD4-CD8-) T cells, and decreased NK, Th17, and regulatory T cell numbers. Notably, the patient harboring the K392R mutation developed T cell LGL leukemia at age 14. Our results broaden the spectrum of phenotypes caused by activating STAT3 mutations, highlight the role of STAT3 in the development and differentiation of multiple immune cell lineages, and strengthen the link between the STAT family of transcription factors and autoimmunity.The signal transducer and activator of transcription (STAT) family of transcription factors orchestrate hematopoietic cell differentiation. Recently, mutations in STAT1, STAT5B, and STAT3 have been linked to development of IPEX-like syndrome. Here, we immunologically characterized three patients with de novo activating mutations in the DNA binding or dimerization domains of STAT3 (p.K392R, p.M394T and p.K658N, respectively). The patients displayed multi-organ autoimmunity, lymphoproliferation, and delayed-onset mycobacterial disease. Immunologically, we noted hypogammaglobulinemia with terminal B cell maturation arrest, dendritic cell deficiency, peripheral eosinopenia, increased double-negative (CD4-CD8-) T cells, and decreased NK, Th17, and regulatory T cell numbers. Notably, the patient harboring the K392R mutation developed T cell LGL leukemia at age 14. Our results broaden the spectrum of phenotypes caused by activating STAT3 mutations, highlight the role of STAT3 in the development and differentiation of multiple immune cell lineages, and strengthen the link between the STAT family of transcription factors and autoimmunity.The signal transducer and activator of transcription (STAT) family of transcription factors orchestrate hematopoietic cell differentiation. Recently, mutations in STAT1, STAT5B, and STAT3 have been linked to development of IPEX-like syndrome. Here, we immunologically characterized three patients with de novo activating mutations in the DNA binding or dimerization domains of STAT3 (p.K392R, p.M394T and p.K658N, respectively). The patients displayed multi-organ autoimmunity, lymphoproliferation, and delayed-onset mycobacterial disease. Immunologically, we noted hypogammaglobulinemia with terminal B cell maturation arrest, dendritic cell deficiency, peripheral eosinopenia, increased double-negative (CD4-CD8-) T cells, and decreased NK, Th17, and regulatory T cell numbers. Notably, the patient harboring the K392R mutation developed T cell LGL leukemia at age 14. Our results broaden the spectrum of phenotypes caused by activating STAT3 mutations, highlight the role of STAT3 in the development and differentiation of multiple immune cell lineages, and strengthen the link between the STAT family of transcription factors and autoimmunity.The signal transducer and activator of transcription (STAT) family of transcription factors orchestrate hematopoietic cell differentiation. Recently, mutations in STAT1, STAT5B, and STAT3 have been linked to development of IPEX-like syndrome. Here, we immunologically characterized three patients with de novo activating mutations in the DNA binding or dimerization domains of STAT3 (p.K392R, p.M394T and p.K658N, respectively). The patients displayed multi-organ autoimmunity, lymphoproliferation, and delayed-onset mycobacterial disease. Immunologically, we noted hypogammaglobulinemia with terminal B cell maturation arrest, dendritic cell deficiency, peripheral eosinopenia, increased double-negative (CD4-CD8-) T cells, and decreased NK, Th17, and regulatory T cell numbers. Notably, the patient harboring the K392R mutation developed T cell LGL leukemia at age 14. Our results broaden the spectrum of phenotypes caused by activating STAT3 mutations, highlight the role of STAT3 in the development and differentiation of multiple immune cell lineages, and strengthen the link between the STAT family of transcription factors and autoimmunity.The signal transducer and activator of transcription (STAT) family of transcription factors orchestrate hematopoietic cell differentiation. Recently, mutations in STAT1, STAT5B, and STAT3 have been linked to development of IPEX-like syndrome. Here, we immunologically characterized three patients with de novo activating mutations in the DNA binding or dimerization domains of STAT3 (p.K392R, p.M394T and p.K658N, respectively). The patients displayed multi-organ autoimmunity, lymphoproliferation, and delayed-onset mycobacterial disease. Immunologically, we noted hypogammaglobulinemia with terminal B cell maturation arrest, dendritic cell deficiency, peripheral eosinopenia, increased double-negative (CD4-CD8-) T cells, and decreased NK, Th17, and regulatory T cell numbers. Notably, the patient harboring the K392R mutation developed T cell LGL leukemia at age 14. Our results broaden the spectrum of phenotypes caused by activating STAT3 mutations, highlight the role of STAT3 in the development and differentiation of multiple immune cell lineages, and strengthen the link between the STAT family of transcription factors and autoimmunity.The signal transducer and activator of transcription (STAT) family of transcription factors orchestrate hematopoietic cell differentiation. Recently, mutations in STAT1, STAT5B, and STAT3 have been linked to development of IPEX-like syndrome. Here, we immunologically characterized three patients with de novo activating mutations in the DNA binding or dimerization domains of STAT3 (p.K392R, p.M394T and p.K658N, respectively). The patients displayed multi-organ autoimmunity, lymphoproliferation, and delayed-onset mycobacterial disease. Immunologically, we noted hypogammaglobulinemia with terminal B cell maturation arrest, dendritic cell deficiency, peripheral eosinopenia, increased double-negative (CD4-CD8-) T cells, and decreased NK, Th17, and regulatory T cell numbers. Notably, the patient harboring the K392R mutation developed T cell LGL leukemia at age 14. Our results broaden the spectrum of phenotypes caused by activating STAT3 mutations, highlight the role of STAT3 in the development and differentiation of multiple immune cell lineages, and strengthen the link between the STAT family of transcription factors and autoimmunity.The signal transducer and activator of transcription (STAT) family of transcription factors orchestrate hematopoietic cell differentiation. Recently, mutations in STAT1, STAT5B, and STAT3 have been linked to development of IPEX-like syndrome. Here, we immunologically characterized three patients with de novo activating mutations in the DNA binding or dimerization domains of STAT3 (p.K392R, p.M394T and p.K658N, respectively). The patients displayed multi-organ autoimmunity, lymphoproliferation, and delayed-onset mycobacterial disease. Immunologically, we noted hypogammaglobulinemia with terminal B cell maturation arrest, dendritic cell deficiency, peripheral eosinopenia, increased double-negative (CD4-CD8-) T cells, and decreased NK, Th17, and regulatory T cell numbers. Notably, the patient harboring the K392R mutation developed T cell LGL leukemia at age 14. Our results broaden the spectrum of phenotypes caused by activating STAT3 mutations, highlight the role of STAT3 in the development and differentiation of multiple immune cell lineages, and strengthen the link between the STAT family of transcription factors and autoimmunity.The signal transducer and activator of transcription (STAT) family of transcription factors orchestrate hematopoietic cell differentiation. Recently, mutations in STAT1, STAT5B, and STAT3 have been linked to development of IPEX-like syndrome. Here, we immunologically characterized three patients with de novo activating mutations in the DNA binding or dimerization domains of STAT3 (p.K392R, p.M394T and p.K658N, respectively). The patients displayed multi-organ autoimmunity, lymphoproliferation, and delayed-onset mycobacterial disease. Immunologically, we noted hypogammaglobulinemia with terminal B cell maturation arrest, dendritic cell deficiency, peripheral eosinopenia, increased double-negative (CD4-CD8-) T cells, and decreased NK, Th17, and regulatory T cell numbers. Notably, the patient harboring the K392R mutation developed T cell LGL leukemia at age 14. Our results broaden the spectrum of phenotypes caused by activating STAT3 mutations, highlight the role of STAT3 in the development and differentiation of multiple immune cell lineages, and strengthen the link between the STAT family of transcription factors and autoimmunity.The signal transducer and activator of transcription (STAT) family of transcription factors orchestrate hematopoietic cell differentiation. Recently, mutations in STAT1, STAT5B, and STAT3 have been linked to development of IPEX-like syndrome. Here, we immunologically characterized three patients with de novo activating mutations in the DNA binding or dimerization domains of STAT3 (p.K392R, p.M394T and p.K658N, respectively). The patients displayed multi-organ autoimmunity, lymphoproliferation, and delayed-onset mycobacterial disease. Immunologically, we noted hypogammaglobulinemia with terminal B cell maturation arrest, dendritic cell deficiency, peripheral eosinopenia, increased double-negative (CD4-CD8-) T cells, and decreased NK, Th17, and regulatory T cell numbers. Notably, the patient harboring the K392R mutation developed T cell LGL leukemia at age 14. Our results broaden the spectrum of phenotypes caused by activating STAT3 mutations, highlight the role of STAT3 in the development and differentiation of multiple immune cell lineages, and strengthen the link between the STAT family of transcription factors and autoimmunity.</p