Microbiota and Immunity during Respiratory Infections: Lung and Gut Affair.

Bacterial and viral respiratory tract infections are the most common infectious diseases, leading to worldwide morbidity and mortality. In the past 10 years, the importance of lung microbiota emerged in the context of pulmonary diseases, although the mechanisms by which it impacts the intestinal environment have not yet been fully identified. On the contrary, gut microbial dysbiosis is associated with disease etiology or/and development in the lung. In this review, we present an overview of the lung microbiome modifications occurring during respiratory infections, namely, reduced community diversity and increased microbial burden, and of the downstream consequences on host-pathogen interaction, inflammatory signals, and cytokines production, in turn affecting the disease progression and outcome. Particularly, we focus on the role of the gut-lung bidirectional communication in shaping inflammation and immunity in this context, resuming both animal and human studies. Moreover, we discuss the challenges and possibilities related to novel microbial-based (probiotics and dietary supplementation) and microbial-targeted therapies (antibacterial monoclonal antibodies and bacteriophages), aimed to remodel the composition of resident microbial communities and restore health. Finally, we propose an outlook of some relevant questions in the field to be answered with future research, which may have translational relevance for the prevention and control of respiratory infections.

Cellular Senescence in Immunity against Infections.

Cellular senescence is characterized by irreversible cell cycle arrest in response to different triggers and an inflammatory secretome. Although originally described in fibroblasts and cell types of solid organs, cellular senescence affects most tissues with advancing age, including the lymphoid tissue, causing chronic inflammation and dysregulation of both innate and adaptive immune functions. Besides its normal occurrence, persistent microbial challenge or pathogenic microorganisms might also accelerate the activation of cellular aging, inducing the premature senescence of immune cells. Therapeutic strategies counteracting the detrimental effects of cellular senescence are being developed. Their application to target immune cells might have the potential to improve immune dysfunctions during aging and reduce the age-dependent susceptibility to infections. In this review, we discuss how immune senescence influences the host's ability to resolve more common infections in the elderly and detail the different markers proposed to identify such senescent cells; the mechanisms by which infectious agents increase the extent of immune senescence are also reviewed. Finally, available senescence therapeutics are discussed in the context of their effects on immunity and against infections.

Cutaneous barrier leakage and gut inflammation drive skin disease in Omenn syndrome.

BACKGROUND: Severe early-onset erythroderma and gut inflammation, with massive tissue infiltration of oligoclonal activated T cells are the hallmark of Omenn syndrome (OS). OBJECTIVE: The impact of altered gut homeostasis in the cutaneous manifestations of OS remains to be clarified. METHODS: We analyzed a cohort of 15 patients with OS and the 129Sv/C57BL/6 knock-in Rag2R229Q/R229Q (Rag2R229Q) mouse model. Homing phenotypes of circulating lymphocytes were analyzed by flow cytometry. Inflammatory cytokines and chemokines were examined in the sera by ELISA and in skin biopsies by immunohistochemistry and in situ RNA hybridization. Experimental colitis was induced in mice by dextran sulfate sodium salt. RESULTS: We show that memory/activated T cells from patients with OS and from the Rag2R229Q mouse model of OS abundantly express the skin homing receptors cutaneous lymphocyte associated antigen and CCR4 (Ccr4), associated with high levels of chemokine C-C motif ligands 17 and 22. Serum levels of LPS are also elevated. A broad Th1/Th2/Th17 inflammatory signature is detected in the periphery and in the skin. Increased Tlr4 expression in the skin of Rag2R229Q mice is associated with enhanced cutaneous inflammation on local and systemic administration of LPS. Likewise, boosting colitis in Rag2R229Q mice results in increased frequency of Ccr4+ splenic T cells and worsening of skin inflammation, as indicated by epidermal thickening, enhanced epithelial cell activation, and dermal infiltration by Th1 effector T cells. CONCLUSIONS: These results support the existence of an interplay between gut and skin that can sustain skin inflammation in OS.

B lymphocytes limit senescence-driven fibrosis resolution and favor hepatocarcinogenesis in mouse liver injury.

Hepatocellular carcinoma (HCC) is a frequent neoplasia and a leading cause of inflammation-related cancer mortality. Despite that most HCCs arise from persistent inflammatory conditions, pathways linking chronic inflammation to cancer development are still incompletely elucidated. We dissected the role of adaptive immunity in the Mdr2 knockout (Mdr2-/- ) mouse, a model of inflammation-associated cancer, in which ablation of adaptive immunity has been induced genetically (Rag2-/- Mdr2-/- and µMt-Mdr2-/- mice) or with in vivo treatments using lymphocyte-specific depleting antibodies (anti-CD20 or anti-CD4/CD8). We found that activated B and T lymphocytes, secreting fibrogenic tumor necrosis factor alpha (TNFα) and other proinflammatory cytokines, infiltrated liver of the Mdr2-/- mice during chronic fibrosing cholangitis. Lymphocyte ablation, in the Rag2-/- Mdr2-/- and µMt-Mdr2-/- mice, strongly suppressed hepatic stellate cell (HSC) activation and extracellular matrix deposition, enhancing HSC transition to cellular senescence. Moreover, lack of lymphocytes changed the intrahepatic metabolic/oxidative state, resulting in skewed macrophage polarization toward an anti-inflammatory M2 phenotype. Remarkably, hepatocarcinogenesis was significantly suppressed in the Rag2-/- Mdr2-/- mice, correlating with reduced TNFα/NF-κB (nuclear factor kappa B) pathway activation. Ablation of CD20+ B cells, but not of CD4+ /CD8+ T cells, in Mdr2-/- mice, promoted senescence-mediated fibrosis resolution and inhibited the protumorigenic TNFα/NF-κB pathway. Interestingly, presence of infiltrating B cells correlated with increased tumor aggressiveness and reduced disease-free survival in human HCC. CONCLUSION: Adaptive immunity sustains liver fibrosis (LF) and favors HCC growth in chronic injury, by modulating innate components of inflammation and limiting the extent of HSC senescence. Therapies designed for B-cell targeting may be an effective strategy in LF. (Hepatology 2018;67:1970-1985).

IL-10 critically modulates B cell responsiveness in Rankl-/- mice.

The immune and the skeletal system are tightly interconnected, and B lymphocytes are uniquely endowed with osteo-interactive properties. In this context, receptor activator of NF-κB (RANK) ligand (RANKL) plays a pivotal role in lymphoid tissue formation and bone homeostasis. Although murine models lacking RANK or RANKL show defects in B cell number, the role of the RANKL-RANK axis on B physiology is still a matter of debate. In this study, we have characterized in detail B cell compartment in Rankl(-/-) mice, finding a relative expansion of marginal zone B cells, B1 cells, and plasma cells associated with increased Ig serum levels, spontaneous germinal center formation, and hyperresponse to CD40 triggering. Such abnormalities were associated with an increased frequency of regulatory B cells and augmented B cell-derived IL-10 production. Remarkably, in vivo IL-10-R blockade reduced T cell-triggered plasma cell differentiation and restrained the expansion of regulatory B cells. These data point to a novel role of the RANKL-RANK axis in the regulation of B cell homeostasis and highlight an unexpected link between IL-10 CD40 signaling and the RANKL pathway.

Anti-CD3ε mAb improves thymic architecture and prevents autoimmune manifestations in a mouse model of Omenn syndrome: therapeutic implications.

Omenn syndrome (OS) is an atypical primary immunodeficiency characterized by severe autoimmunity because of activated T cells infiltrating target organs. The impaired recombinase activity in OS severely affects expression of the pre-T-cell receptor complex in immature thymocytes, which is crucial for an efficient development of the thymic epithelial component. Anti-CD3ε monoclonal antibody (mAb) treatment in RAG2(-/-) mice was previously shown to mimic pre-TCR signaling promoting thymic expansion. Here we show the effect of anti-CD3ε mAb administration in the RAG2(R229Q) mouse model, which closely recapitulates human OS. These animals, in spite of the inability to induce the autoimmune regulator, displayed a significant amelioration in thymic epithelial compartment and an important reduction of peripheral T-cell activation and tissue infiltration. Furthermore, by injecting a high number of RAG2(R229Q) progenitors into RAG2(-/-) animals previously conditioned with anti-CD3ε mAb, we detected autoimmune regulator expression together with the absence of peripheral immunopathology. These observations indicate that improving epithelial thymic function might ameliorate the detrimental behavior of the cell-autonomous RAG defect. Our data provide important therapeutic proof of concept for future clinical applications of anti-CD3ε mAb treatment in severe combined immunodeficiency forms characterized by poor thymus function and autoimmunity.

The role of WNT and IL-1 signaling in osteoarthritis: therapeutic implications for platelet-rich plasma therapy.

Different from inflammatory arthritis, where biologicals and targeted synthetic molecules have revolutionized the disease course, no drug has demonstrated a disease modifying activity in osteoarthritis, which remains one of the most common causes of disability and chronic pain worldwide. The pharmacological therapy of osteoarthritis is mainly directed towards symptom and pain relief, and joint replacement is still the only curative strategy. Elucidating the disease pathophysiology is essential to understand which mechanisms can be targeted by innovative therapies. It has extensively been demonstrated that aberrant WNT and IL-1 signaling pathways are responsible for cartilage degeneration, impaired chondrocyte metabolism and differentiation, increased extracellular matrix degradation, and altered subchondral bone homeostasis. Platelet-rich plasma is an autologous blood derivative containing a concentration of platelets that is much higher than the whole blood counterpart and has shown promising results in the treatment of early knee osteoarthritis. Among the proposed mechanisms, the modulation of WNT and IL-1 pathways is of paramount importance and is herein reviewed in light of the proposed regenerative approaches.

Spatial resolution of cellular senescence dynamics in human colorectal liver metastasis.

Hepatic metastasis is a clinical challenge for colorectal cancer (CRC). Senescent cancer cells accumulate in CRC favoring tumor dissemination. Whether this mechanism progresses also in metastasis is unexplored. Here, we integrated spatial transcriptomics, 3D-microscopy, and multicellular transcriptomics to study the role of cellular senescence in human colorectal liver metastasis (CRLM). We discovered two distinct senescent metastatic cancer cell (SMCC) subtypes, transcriptionally located at the opposite pole of epithelial (e) to mesenchymal (m) transition. SMCCs differ in chemotherapy susceptibility, biological program, and prognostic roles. Mechanistically, epithelial (e)SMCC initiation relies on nucleolar stress, whereby c-myc dependent oncogene hyperactivation induces ribosomal RPL11 accumulation and DNA damage response. In a 2D pre-clinical model, we demonstrated that RPL11 co-localized with HDM2, a p53-specific ubiquitin ligase, leading to senescence activation in (e)SMCCs. On the contrary, mesenchymal (m)SMCCs undergo TGFß paracrine activation of NOX4-p15 effectors. SMCCs display opposing effects also in the immune regulation of neighboring cells, establishing an immunosuppressive environment or leading to an active immune workflow. Both SMCC signatures are predictive biomarkers whose unbalanced ratio determined the clinical outcome in CRLM and CRC patients. Altogether, we provide a comprehensive new understanding of the role of SMCCs in CRLM and highlight their potential as new therapeutic targets to limit CRLM progression.

Genetically determined lymphopenia and autoimmune manifestations.

Hypomorphic defects of V(D)J recombination in humans lead to residual T cell development. In these lymphopenic conditions, homeostatic lymphocyte proliferation occurs, and key mechanisms that normally maintain host tolerance are altered, allowing peripheral expansion of oligoclonal and autoreactive T cells. Recently described murine models support this notion. This review describes human and murine situations, in which genetically determined T and B cell lymphopenia is associated with autoimmune manifestations.

Human peripheral lymphoid tissues contain autoimmune regulator-expressing dendritic cells.

Autoimmune regulator (AIRE) modulates the expression of tissue-restricted antigens (TSAs) and promotes central tolerance in the thymus. However, few autoreactive T cells escape negative selection and reach the periphery, where peripheral tolerance is required to avoid autoimmunity. Murine lymph nodes (LNs) have been shown to contain "stromal" cells expressing AIRE and TSAs. Here we report the occurrence of AIRE-expressing cells in human peripheral lymphoid tissues, including LNs, tonsils, and gut-associated lymphoid tissue, with the exception of the spleen. Notably, AIRE+ cells are absent in fetal LNs and, in postnatal life, they are more numerous in abdominal than in superficial LNs, thus suggesting that their development in periphery may depend on instructive signals from microenvironment and antigen challenge. Extrathymic AIRE+ cells show a dendritic morphology, consistently express human leukocyte antigen-DR (HLADR) and fascin, and are largely positive for CD11c and S100 and for the dendritic cell-activation markers CD40, CD83, DC-LAMP/CD208, and CCR7. Lymphoid, myelomonocytic, mesenchymal, and epithelial cell lineage markers are negative. The HLADRhigh/AIRE+ cell fraction isolated from mesenteric LNs expressed TSAs (insulin, CYP17A1, and CYP21A2), as well as molecules associated with tolerogenic functions, such as interleukin-10 and indoleamine 2,3-dioxygenase. Data indicate that AIRE+ cells in human peripheral lymphoid tissues correspond to a subset of activated interdigitating dendritic cells expressing TSAs and the tolerogenic molecules indoleamine 2,3-dioxygenase and interleukin-10, suggestive of a potential tolerogenic function.

Defect of regulatory T cells in patients with Omenn syndrome.

BACKGROUND: Omenn syndrome (OS) is an autosomal-recessive disorder characterized by severe immunodeficiency and T-cell-mediated autoimmunity. The disease is caused by hypomorphic mutations in recombination-activating genes that hamper the process of Variable (V) Diversity (D) Joining (J) recombination, leading to the generation of autoreactive T cells. We have previously shown that in OS the expression of autoimmune regulator, a key factor governing central tolerance, is markedly reduced. OBJECTIVE: Here, we have addressed the role of peripheral tolerance in the disease pathogenesis. METHODS: We have analyzed forkhead box protein P3 (FOXP3) expression in peripheral blood T cells of 4 patients with OS and in lymphoid organs of 8 patients with OS and have tested the suppressive activity of sorted CD4(+) CD25(high) peripheral blood T cells in 2 of these patients. RESULTS: We have observed that CD4(+)CD25(high)T cells isolated ex vivo from patients with OS failed to suppress proliferation of autologous or allogenic CD4(+) responder T cells. Moreover, despite individual variability in the fraction of circulating FOXP3(+) CD4 cells in patients with OS, the immunohistochemical analysis of FOXP3 expression in lymph nodes and thymus of patients with OS demonstrated a severe reduction of this cell subset compared with control tissues. CONCLUSION: Overall, these results suggest a defect of regulatory T cells in OS leading to a breakdown of peripheral tolerance, which may actively concur to the development of autoimmune manifestations in the disease.

A hypomorphic R229Q Rag2 mouse mutant recapitulates human Omenn syndrome.

Rag enzymes are the main players in V(D)J recombination, the process responsible for rearrangement of TCR and Ig genes. Hypomorphic Rag mutations in humans, which maintain partial V(D)J activity, cause a peculiar SCID associated with autoimmune-like manifestations, Omenn syndrome (OS). Although a deficient ability to sustain thymopoiesis and to produce a diverse T and B cell repertoire explains the increased susceptibility to severe infections, the molecular and cellular mechanisms underlying the spectrum of clinical and immunological features of OS remain poorly defined. In order to better define the molecular and cellular pathophysiology of OS, we generated a knockin murine model carrying the Rag2 R229Q mutation previously described in several patients with OS and leaky forms of SCID. These Rag2(R229Q/R229Q) mice showed oligoclonal T cells, absence of circulating B cells, and peripheral eosinophilia. In addition, activated T cells infiltrated gut and skin, causing diarrhea, alopecia, and, in some cases, severe erythrodermia. These findings were associated with reduced thymic expression of Aire and markedly reduced numbers of naturally occurring Tregs and NKT lymphocytes. In conclusion, Rag2(R229Q/R229Q) mice mimicked most symptoms of human OS; our findings support the notion that impaired immune tolerance and defective immune regulation are involved in the pathophysiology of OS.

AIRE deficiency in thymus of 2 patients with Omenn syndrome.

Omenn syndrome is a severe primary immunodeficiency with putative autoimmune manifestations of the skin and gastrointestinal tract. The disease is caused by hypomorphic mutations in recombination-activating genes that impair but do not abolish the process of VDJ recombination, leading to the generation of autoreactive T cells with a highly restricted receptor repertoire. Loss of central tolerance in genetically determined autoimmune diseases, e.g., autoimmune polyendocrinopathy-candidiasis-ectodermal dystrophy, is associated with defective expression by medullary thymic epithelial cells of AIRE, the transcription activator that induces thymic expression of tissue-specific antigens. Analysis of AIRE expression in the thymi of 2 Omenn syndrome patients and 1 SCID patient, by real-time RT-PCR and immunohistochemistry, demonstrated a profound reduction in the levels of AIRE mRNA and protein in patients as compared with a normal control subject. Lack of AIRE was associated with normal or even increased levels of keratin and lymphotoxin-beta receptor mRNAs, while mRNAs of the self-antigens insulin, cytochrome P450 1a2, and fatty acid-binding protein were undetectable in thymi from immunodeficiency patients. These results demonstrate that deficiency of AIRE expression is observed in severe immunodeficiencies characterized by abnormal T cell development and suggest that in Omenn syndrome, the few residual T cell clones that develop may escape negative selection and thereafter expand in the periphery, causing massive autoimmune reactions.

RAG-dependent primary immunodeficiencies.

Mutations in recombination activating genes 1 and 2 (RAG1 and RAG2) cause a spectrum of severe immunodeficiencies ranging from classical T cell-B cell-severe combined immunodeficiency (T(-)B(-)SCID) and Omenn syndrome (OS) to an increasing number of peculiar cases. While it is well established from biochemical data that the specific genetic defect in either of the RAG genes is the first determinant of the clinical presentation, there is also increasing evidence that environmental factors play an important role and can lead to a different phenotypic expression of a given genotype. However, a better understanding of the mechanisms by which the molecular defect impinges on the cellular phenotype of OS is still lacking. Ongoing studies in knock-in mice could better clarify this aspect.

Tissue-specific sensitivity to AID expression in transgenic mouse models.

Activation-induced cytidine deaminase (AID), an enzyme with homology to members of the APOBEC family, is involved in somatic hypermutation (SHM) of immunoglobulin (Ig) genes, either by direct deamination of DNA or by an indirect action through its putative RNA editing activity. AID is able to mutate both Ig-like reporter constructs and selected non-Ig genes in normal B cells and in other cells when ectopically overexpressed in mammalian cells and transgenic mice. However, in spite of the fact that in these transgenic animals AID activity was driven by an ubiquitous promoter, only T lymphomas and lung adenomas occurred. In the present work, we constructed three sets of transgenic mice in which AID was under the control of lck, HTLV-I and MMTV promoters, respectively. The lck/AID mice developed thymic lymphomas with variable but high efficiency, while no tumor was detected in HTLV-I/AID mice after two years of monitoring. Four MMTV/AID founder mice died with an atypical clinical picture, although no mammary tumor was found. These findings suggest that additional factors, present in thymocytes but not in other tissues or in lymphoid cells at different stages of differentiation, are needed for AID to fully manifest its tumorigenic potential in mouse. Alternatively, the display of full AID mutagenic and transforming activity could be related to the existence of physiologic DSBs which occur in both thymocytes and switching B cells.

Rag defects and thymic stroma: lessons from animal models.

Thymocytes and thymic epithelial cells (TECs) cross-talk is essential to support T cell development and preserve thymic architecture and maturation of TECs and Foxp3(+) natural regulatory T cells. Accordingly, disruption of thymic lymphostromal cross-talk may have major implications on the thymic mechanisms that govern T cell tolerance. Several genetic defects have been described in humans that affect early stages of T cell development [leading to severe combined immune deficiency (SCID)] or late stages in thymocyte maturation (resulting in combined immunodeficiency). Hypomorphic mutations in SCID-causing genes may allow for generation of a limited pool of T lymphocytes with a restricted repertoire. These conditions are often associated with infiltration of peripheral tissues by activated T cells and immune dysregulation, as best exemplified by Omenn syndrome (OS). In this review, we will discuss our recent findings on abnormalities of thymic microenvironment in OS with a special focus of defective maturation of TECs, altered distribution of thymic dendritic cells and impairment of deletional and non-deletional mechanisms of central tolerance. Here, taking advantage of mouse models of OS and atypical SCID, we will discuss how modifications in stromal compartment impact and shape lymphocyte differentiation, and vice versa how inefficient T cell signaling results in defective stromal maturation. These findings are instrumental to understand the extent to which novel therapeutic strategies should act on thymic stroma to achieve full immune reconstitution.

Hypomorphic mutation in the RAG2 gene affects dendritic cell distribution and migration.

OS is a severe combined immunodeficiency characterized by erythrodermia and protracted diarrhea as a result of infiltration of oligoclonal-activated T cells, caused by hypomorphic mutations in RAGs. The RAG2(R229Q) mouse model fully recapitulates the clinical OS phenotype. We evaluated whether T and B cell defects, together with the abnormal lymphoid structure, could affect DC homeostasis and function. High density of LCs was observed in skin biopsies of Omenn patients and in the derma of RAG2(R229Q) mice, correlating with the presence of erythrodermia. In vivo models of cutaneous skin painting and CHS demonstrated a decreased migration of RAG2(R229Q) DCs-in particular, LCs-into draining LNs. Interestingly, at steady state, RAG2(R229Q) mice showed a reduction in DC number in all hematopoietic organs except LNs. Analysis of the MHCII marker revealed a diminished expression also upon the LPS-driven inflammatory condition. Despite the decreased number of peripheral DCs, BM pre-cDCs were present in normal number compared with RAG2(+/+) controls, whereas pDCs and monocytes were reduced significantly. Overall, these results point to a secondary defect in the DC compartment, which contributes to clinical manifestations and autoimmunity in OS.

RANK-dependent autosomal recessive osteopetrosis: characterization of five new cases with novel mutations.

Autosomal recessive osteopetrosis (ARO) is a genetically heterogeneous disorder attributed to reduced bone resorption by osteoclasts. Most human AROs are classified as osteoclast rich, but recently two subsets of osteoclast-poor ARO have been recognized as caused by defects in either TNFSF11 or TNFRSF11A genes, coding the RANKL and RANK proteins, respectively. The RANKL/RANK axis drives osteoclast differentiation and also plays a role in the immune system. In fact, we have recently reported that mutations in the TNFRSF11A gene lead to osteoclast-poor osteopetrosis associated with hypogammaglobulinemia. Here we present the characterization of five additional unpublished patients from four unrelated families in which we found five novel mutations in the TNFRSF11A gene, including two missense and two nonsense mutations and a single-nucleotide insertion. Immunological investigation in three of them showed that the previously described defect in the B cell compartment was present only in some patients and that its severity seemed to increase with age and the progression of the disease. HSCT performed in all five patients almost completely cured the disease even when carried out in late infancy. Hypercalcemia was the most important posttransplant complication. Overall, our results further underline the heterogeneity of human ARO also deriving from the interplay between bone and the immune system, and highlight the prognostic and therapeutic implications of the molecular diagnosis.

Osteopetrosis rescue upon RANKL administration to Rankl(-/-) mice: a new therapy for human RANKL-dependent ARO.

In the last decades the molecular basis of monogenic diseases has been largely unraveled, although their treatment has often remained unsatisfactory. Autosomal recessive osteopetrosis (ARO) belongs to the small group of genetic diseases that are usually treated with hematopoietic stem cell transplantation (HSCT). However, this approach is not effective in the recently identified form carrying mutations in the receptor activator of NF-κB ligand (RANKL) gene. In this subset, therapy replacement approach based on RANKL delivery has a strong rationale. Here we demonstrate that the systematic administration of RANKL for 1 month to Rankl(-/-) mice, which closely resemble the human disease, significantly improves the bone phenotype and has beneficial effects on bone marrow, spleen and thymus; major adverse effects arise only when mice are clearly overtreated. Overall, we provide evidence that the pharmacological administration of RANKL represents the appropriate treatment option for RANKL-deficient ARO patients, to be validated in a pilot clinical trial.

Omenn syndrome does not live by V(D)J recombination alone.

PURPOSE OF REVIEW: During the past decade, easy access to sequence analyses has allowed us to increase our understanding of the pathogenesis of severe combined immunodeficiencies. Here, we describe the expanding clinical and immunological spectrum associated with Omenn syndrome phenotype. In particular, we review the cellular and molecular mechanisms involved in the pathophysiology of 'classical' Omenn syndrome due to the recombination activating gene (RAG) defects and of a new subgroup of Omenn-like disorders. RECENT FINDINGS: Different types of mutations are associated with the Omenn phenotype characterized by skin erythroderma, oligoclonal-activated T cells and elevated IgE in the absence of circulating B cells. Extensive studies conducted over the last few years have allowed the definition of the 'classical form' of Omenn syndrome due to hypomorphic defects in genes involved in V(D)J recombination, mainly RAG genes, and 'Omenn-like' features associated with mutations in genes involved in the maturation steps of lymphoid cells other than V(D)J recombination. Moreover, an increasing number of diseases other than those due to V(D)J recombination defects develop Omenn signs. SUMMARY: Impaired but not abolished V(D)J recombination process leads to the generation of a few T cells which expand in the periphery, infiltrate target organs such as skin and gut, resulting in severe erythroderma and colitis, both typical signs of Omenn syndrome. Extensive molecular studies now demonstrate that genes other than V(D)J molecules have a role in the pathogenesis of this disease, supporting the evidence that 'Omenn' defines an inflammatory condition associated with various genetic defects.