FOXN1 deficiency is a primary immunodeficiency characterized by athymia, alopecia totalis, and nail dystrophy. Two infants with FOXN1 deficiency were transplanted with cultured postnatal thymus tissue. Subject 1 presented with disseminated Bacillus Calmette-Guérin infection and oligoclonal T cells with no naive markers. Subject 2 had respiratory failure, human herpes virus 6 infection, cytopenias, and no circulating T cells. The subjects were given thymus transplants at 14 and 9 months of life, respectively. Subject 1 received immunosuppression before and for 10 months after transplantation. With follow up of 4.9 and 2.9 years, subjects 1 and 2 are well without infectious complications. The pretransplantation mycobacterial disease in subject 1 and cytopenias in subject 2 resolved. Subject 2 developed autoimmune thyroid disease 1.6 years after transplantation. Both subjects developed functional immunity. Subjects 1 and 2 have 1053/mm3 and 1232/mm3 CD3+ cells, 647/mm3 and 868/mm3 CD4+ T cells, 213/mm3 and 425/mm3 naive CD4+ T cells, and 10 200 and 5700 T-cell receptor rearrangement excision circles per 100 000 CD3+ cells, respectively. They have normal CD4 T-cell receptor β variable repertoires. Both subjects developed antigen-specific proliferative responses and have discon-tinued immunoglobulin replacement. In summary, thymus transplantation led to T-cell reconstitution and function in these FOXN1 deficient infants.

IL-17 is a pro-inflammatory cytokine implicated in autoimmune and inflammatory conditions. The development/survival of IL-17-producing CD4 T cells (Th17) share critical cues with B-cell differentiation and the circulating follicular T helper subset was recently shown to be enriched in Th17 cells able to help B-cell differentiation. We investigated a putative link between Th17-cell homeostasis and B cells by studying the Th17-cell compartment in primary B-cell immunodeficiencies. Common Variable Immunodeficiency Disorders (CVID), defined by defects in B-cell differentiation into plasma and memory B cells, are frequently associated with autoimmune and inflammatory manifestations but we found no relationship between these and Th17-cell frequency. In fact, CVID patients showed a decrease in Th17-cell frequency in parallel with the expansion of activated non-differentiated B cells (CD21lowCD38low). Moreover, Congenital Agammaglobulinemia patients, lacking B cells due to impaired early B-cell development, had a severe reduction of circulating Th17 cells. Finally, we found a direct correlation in healthy individuals between circulating Th17-cell frequency and both switched-memory B cells and serum BAFF levels, a crucial cytokine for B-cell survival. Overall, our data support a relationship between Th17-cell homeostasis and B-cell maturation, with implications for the understanding of the pathogenesis of inflammatory/autoimmune diseases and the physiology of B-cell depleting therapies.

Summary WHIM syndrome (warts, hypogammaglobulinemia, infections, and myelokathexis), a primary immunodeficiency disorder involving panleukopenia, is caused by autosomal dominant gain-of-function mutations in CXC chemokine receptor 4 (CXCR4). Myelokathexis is neutropenia caused by neutrophil retention in bone marrow. Patients with WHIM syndrome are often treated with granulocyte colony-stimulating factor (G-CSF), which can increase neutrophil counts but does not affect cytopenias other than neutropenia. In this investigator-initiated, open-label study, three severely affected patients with WHIM syndrome who could not receive G-CSF were treated with low-dose plerixafor, a CXCR4 antagonist, for 19 to 52 months. Myelofibrosis, panleukopenia, anemia, and thrombocytopenia were ameliorated, the wart burden and frequency of infection declined, human papillomavirus–associated oropharyngeal squamous-cell carcinoma stabilized, and quality of life improved markedly. Adverse events were mainly infections attributable to the underlying immunodeficiency. One patient died from complications of elective reconstructive surgery. (Funded by the National Institutes of Health.)

Naïve FoxP3-expressing regulatory T-cells (Tregs) are essential to control immune responses via continuous replenishment of the activated-Treg pool with thymus-committed suppressor cells. The mechanisms underlying naïve-Treg maintenance throughout life in face of the age-associated thymic involution remain unclear. We found that in adults thymectomized early in infancy the naïve-Treg pool is remarkably well preserved, in contrast to conventional naïve CD4 T-cells. Naïve-Tregs featured high levels of cycling and pro-survival markers, even in healthy individuals, and contrasted with other circulating naïve/memory CD4 T-cell subsets in terms of their strong γc-cytokine-dependent signaling, particularly in response to IL-7. Accordingly, ex-vivo stimulation of naïve-Tregs with IL-7 induced robust cytokine-dependent signaling, Bcl-2 expression, and phosphatidylinositol 3-kinase (PI3K)-dependent proliferation, whilst preserving naïve phenotype and suppressive capacity. Altogether, our data strongly implicate IL-7 in the thymus-independent long-term survival of functional naïve-Tregs, and highlight the potential of targeting the IL-7 pathway to modulate Tregs in different clinical settings.

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Forkhead box N1 (FOXN1) is a transcription factor crucial for thymic epithelium development and prevention of its involution. Investigation of a patient with a rare homozygous FOXN1 mutation (R255X), leading to alopecia universalis and thymus aplasia, unexpectedly revealed non-maternal circulating T-cells, and, strikingly, large numbers of aberrant double-negative αβ T-cells (CD4negCD8neg, DN) and regulatory-like T-cells. These data raise the possibility that a thymic rudiment persisted, allowing T-cell development, albeit with disturbances in positive/negative selection, as suggested by DN and FoxP3+ cell expansions. Although regulatory-like T-cell numbers normalized following HLA-mismatched thymic transplantation, the αβDN subset persisted 5 years post-transplantation. Involution of thymus allograft likely occurred 3 years post-transplantation based on sj/βTREC ratio, which estimates intrathymic precursor T-cell divisions and, consequently, thymic explant output. Nevertheless, functional immune-competence was sustained, providing new insights for the design of immunological reconstitution strategies based on thymic transplantation, with potential applications in other clinical settings.

Naive CD4 T-cell maintenance is critical for immune competence. We investigated here the fine-tuning of homeostatic mechanisms of the naive compartment to counteract the loss of de novo CD4 T-cell generation. Adults thymectomized in early childhood during corrective cardiac surgery were grouped based on presence or absence of thymopoiesis and compared with age-matched controls. We found that the preservation of the CD31− subset was independent of the thymus and that its size is tightly controlled by peripheral mechanisms, including prolonged cell survival as attested by Bcl-2 levels. Conversely, a significant contraction of the CD31+ naive subset was observed in the absence of thymic activity. This was associated with impaired responses of purified naive CD4 T-cells to IL-7, namely, in vitro proliferation and upregulation of CD31 expression, which likely potentiated the decline in recent thymic emigrants. Additionally, we found no apparent constraint in the differentiation of naive cells into the memory compartment in individuals completely lacking thymic activity despite upregulation of DUSP6, a phosphatase associated with increased TCR threshold. Of note, thymectomized individuals featuring some degree of thymopoiesis were able to preserve the size and diversity of the naive CD4 compartment, further arguing against complete thymectomy in infancy. Overall, our data suggest that robust peripheral mechanisms ensure the homeostasis of CD31− naive CD4 pool and point to the requirement of continuous thymic activity to the maintenance of IL-7-driven homeostatic proliferation of CD31+ naive CD4 T-cells, which is essential to secure T-cell diversity throughout life.

The contribution of the decline in thymic activity for the emergence of autoimmunity is still debatable. Immune-competent adults submitted to complete thymectomy early in life provide a unique model to address this question. We applied here strict criteria to identify adults lacking thymic activity based on sjTREC levels, to exclude thymic rebound and/or ectopic thymuses. In agreement, they featured severe naïve CD4 T-cell depletion and contraction of T-cell receptor diversity. Notwithstanding this, there was neither increased incidence of autoimmune disease in comparison with age-matched controls nor significant changes in their IgG/IgA/IgM/IgE autoreactivity profiles, as assessed through extensive arrays. We reasoned that the observed relative preservation of the regulatory T-cell compartment, including maintenance of naïve regulatory CD4 T-cells, may contribute to limit the emergence of autoimmunity upon thymectomy. Our findings have implications in other clinical settings with impaired thymic activity, and are particularly relevant to studies of autoimmunity in ageing.