Circulating CD1c+ myeloid dendritic cells are potential precursors to LCH lesion CD1a+CD207+ cells

Langerhans Cell Histiocytosis and Other Histiocytic Lesions

BACKGROUND

Histiocytoses, including Langerhans cell histiocytosis (LCH), comprise a diverse group of histiocytic disorders characterized by the abnormal accumulation and proliferation of histiocytes in various tissues or organs throughout the body, ranging from benign, self-limited conditions to aggressive malignancies and systemic inflammatory syndromes. These lesions present unique diagnostic challenges due to their broad spectrum of clinical presentations, overlapping histopathological and immunophenotypical features, and genetic complexity.

METHODS

This review analyzes major histiocytic lesions, focusing on their epidemiology, clinical presentations, histologic and immunophenotypic features, and genetic characteristics to facilitate accurate diagnosis and differentiation among these histiocytoses.

RESULTS

LCH, a well-recognized lesion, can affect various organ systems and necessitates differentiation from other types of histiocytoses such as Erdheim-Chester disease (ECD), Rosai-Dorfman-Destombes disease (RDD), and cutaneous and mucocutaneous non-Langerhans cell histiocytoses. Some histiocytic lesions, such as histiocytic sarcoma, are inherently malignant, while others, like hemophagocytic lymphohistiocytosis (HLH), manifest as severe, potentially life-threatening systemic inflammatory syndromes. Recent molecular genetic studies revealed recurrent genetic alterations in the MAPK pathway, such as BRAF V600E and MAP2K1 in LCH and ECD, and KRAS, NRAS, and MAP2K1 mutations in a subset of RDD. Malignant histiocytoses frequently show alterations in tumor suppressor genes like TP53 and CDKN2A.

CONCLUSION

Precise classification of histiocytic lesions relies on a comprehensive diagnostic approach that integrates clinical, histologic, immunophenotypic, and genetic data. Recent genetic advances shed light on these conditions' unique but occasionally overlapping pathogenic mechanisms. Molecular genetics advancements continue to refine diagnostic accuracy and present new therapeutic targets, especially for aggressive or treatment-resistant cases.

Identification of immune-related hub genes and potential molecular mechanisms involved in COVID-19 via integrated bioinformatics analysis

COVID-19, caused by the SARS-CoV-2 virus, poses significant health challenges worldwide, particularly due to severe immune-related complications. Understanding the molecular mechanisms and identifying key immune-related genes (IRGs) involved in COVID-19 pathogenesis is critical for developing effective prevention and treatment strategies. This study employed computational tools to analyze biological data (bioinformatics) and a method for inferring causal relationships based on genetic variations, known as Mendelian randomization (MR), to explore the roles of IRGs in COVID-19. We identified differentially expressed genes (DEGs) from datasets available in the Gene Expression Omnibus (GEO), comparing COVID-19 patients with healthy controls. IRGs were sourced from the ImmPort database. We conducted functional enrichment analysis, pathway analysis, and immune infiltration assessments to determine the biological significance of the identified IRGs. A total of 360 common differential IRGs were identified. Among these genes, CD1C, IL1B, and SLP1 have emerged as key IRGs with potential protective effects against COVID-19. Pathway enrichment analysis revealed that CD1C is involved in terpenoid backbone biosynthesis and Th17 cell differentiation, while IL1B is linked to B-cell receptor signaling and the NF-kappa B signaling pathway. Significant correlations were observed between key genes and various immune cells, suggesting that they influence immune cell modulation in COVID-19. This study provides new insights into the immune mechanisms underlying COVID-19, highlighting the crucial role of IRGs in disease progression. These findings suggest that CD1C and IL1B could be potential therapeutic targets. The integrated bioinformatics and MR analysis approach offers a robust framework for further exploring immune responses in COVID-19 patients, as well as for targeted therapy and vaccine development.

BRAF-V600E mutations in plasma and peripheral blood mononuclear cells correlate with prognosis of pediatric Langerhans cell histiocytosis treated with first-line therapy

BACKGROUND

The clinical relevance of BRAF-V600E alleles in peripheral blood mononuclear cells (PBMCs) and the prognostic impact of the mutants in cell-free (cf) and PBMC DNAs of Langerhans cell histiocytosis (LCH) have not been fully clarified in pediatric LCH.

METHODS

We retrospectively determined the levels of BRAF-V600E mutation in paired plasma and PBMC samples at the time of diagnosis of LCH. Subsequently, we performed a separate or combined analysis of the clinical and prognostic impact of the mutants.

RESULTS

We assessed BRAF-V600E mutation in peripheral blood from 94 patients of childhood LCH. Our data showed that cfBRAF-V600E was related to young age, multiple-system (MS) disease, involvements of organs with high risk, increased risk of relapse, and worse progression-free survival (PFS) of patients. We also observed that the presence of BRAF-V600E in PBMCs at baseline was significantly associated with MS LCH with risk organ involvement, younger age, and disease progression or relapse. The coexisting of plasma(+)/PBMC(+) identified 36.2% of the patients with the worst outcome, and the hazard ratio was more significant than either of the two alone or neither, indicating that combined analysis of the mutation in plasma and PBMCs was more accurate to predict relapse than evaluation of either one.

CONCLUSIONS

Concurrent assessment of BRAF-V600E mutation in plasma and PBMCs significantly impacted the prognosis of children with LCH. Further prospective studies with larger cohorts need to validate the results of this study.

Dabrafenib and trametinib in Langerhans cell histiocytosis and other histiocytic disorders

The standard treatment for Langerhans cell histiocytosis (LCH) is chemotherapy, although the failure rates are high. Since MAP-kinase activating mutations are found in most cases, BRAF- and MEK-inhibitors have been used successfully to treat patients with refractory or relapsed disease. However, data on long-term responses in children are limited and there are no data on the use of these inhibitors as first-line therapy. We treated 34 patients (26 with LCH, 2 with juvenile xanthogranuloma, 2 with Rosai-Dorfman disease, and 4 with presumed single site-central nervous system histiocytosis) with dabrafenib and/or trametinib, either as first line or after relapse or failure of chemotherapy. Sixteen patients, aged 1.3-21 years, had disease that was recurrent or refractory to chemotherapy, nine of whom had multisystem LCH with risk-organ involvement. With a median treatment duration of 4.3 years, 15 (94%) patients have sustained favorable responses. Eighteen patients, aged 0.2-45 years, received an inhibitor as first-line treatment. All of these have had sustained favorable responses, with a median treatment duration of 2.5 years. Three patients with presumed isolated central nervous system/pituitary stalk histiocytosis had stabilization or improvement of their disease. Overall, inhibitors were well tolerated. Five patients with single-system LCH discontinued therapy and remain off therapy without recurrence. In contrast, all four patients with multisystem disease who discontinued therapy had to restart treatment. Our data suggest that children suffering from histiocytoses can be treated safely and effectively with dabrafenib or trametinib. Additional studies are, however, needed to determine the long-term safety and optimal duration of therapy.

Langerhans cell histiocytosis: current advances in molecular pathogenesis

Langerhans cell histiocytosis (LCH) is a rare and clinically heterogeneous hematological disease characterized by the accumulation of mononuclear phagocytes in various tissues and organs. LCH is often characterized by activating mutations of the mitogen-activated protein kinase (MAPK) pathway with BRAFV600E being the most recurrent mutation. Although this discovery has greatly helped in understanding the disease and in developing better investigational tools, the process of malignant transformation and the cell of origin are still not fully understood. In this review, we focus on the newest updates regarding the molecular pathogenesis of LCH and novel suggested pathways with treatment potential.

Lineage switching of the cellular distribution of BRAFV600E in multisystem Langerhans cell histiocytosis

Most children with high-risk Langerhans cell histiocytosis (LCH) have BRAFV600E mutation. BRAFV600E alleles are detectable in myeloid mononuclear cells at diagnosis but it is not known if the cellular distribution of mutation evolves over time. Here, the profiles of 16 patients with high-risk disease were analyzed. Two received conventional salvage chemotherapy, 4 patients on inhibitors were tracked at intervals of 3 to 6 years, and 10 patients, also given inhibitors, were analyzed more than 2 years after diagnosis. In contrast to the patients responding to salvage chemotherapy who completely cleared BRAFV600E within 6 months, children who received inhibitors maintained high BRAFV600E alleles in their blood. At diagnosis, mutation was detected predominantly in monocytes and myeloid dendritic cells. With time, mutation switched to the T-cell compartment, which accounted for most of the mutational burden in peripheral blood mononuclear cells, more than 2 years from diagnosis (median, 85.4%; range, 44.5%-100%). The highest level of mutation occurred in naïve CD4+ T cells (median, 51.2%; range, 3.8%-93.5%). This study reveals an unexpected lineage switch of BRAFV600E mutation in high-risk LCH, which may influence monitoring strategies for the potential withdrawal of inhibitor treatment and has new implications for the pathogenesis of neurodegeneration, which occurred in 4 patients.

CD1C is associated with breast cancer prognosis and immune infiltrates

BACKGROUND

The tumor microenvironment (TME) in breast cancer plays a vital role in occurrence, development, and therapeutic responses. However, immune and stroma constituents in the TME are major obstacles to understanding and treating breast cancer. We evaluated the significance of TME-related genes in breast cancer.

METHODS

Invasive breast cancer (BRCA) samples were retrieved from the TCGA and GEO databases. Stroma and immune scores of samples as well as the proportion of tumor infiltrating immune cells (TICs) were calculated using the ESTIMATE and CIBERSORT algorithms. TME-related differentially expressed genes (DEGs) were analyzed by a protein interaction (PPI) network and univariate Cox regression to determine CD1C as a hub gene. Subsequently, the prognostic value of CD1C, its response to immunotherapy, and its mechanism in the TME were further studied.

RESULTS

In BRCA, DEGs were determined to identify CD1C as a hub gene. The expression level of CD1C in BRCA patients was verified based on the TCGA database, polymerase chain reaction (PCR) results, and western blot analysis. Immunohistochemical staining (IHC) results revealed a correlation between prognosis, clinical features, and CD1C expression in BRCA. Enrichment analysis of GSEA and GSVA showed that CD1C participates in immune-associated signaling pathways. CIBERSORT showed that CD1C levels were associated with tumor immune infiltrating cells (TILs), such as different kinds of T cells. Gene co-expression analysis showed that CD1C and the majority of immune-associated genes were co-expressed in BRCA. In renal cell carcinoma, patients with a high expression of CD1C had a better immunotherapy effect.

CONCLUSION

CD1C is an important part of the TME and participates in immune activity regulation in breast tumors. CD1C is expected to become a prognostic marker and a new treatment target for breast cancer.

Detection of Immune Microenvironment Changes and Immune-Related Regulators in Langerhans Cell Histiocytosis Bone Metastasis

The inflammation/immune response pathway is considered a key contributor to the development of Langerhans cell histiocytosis (LCH) bone metastasis. However, the dynamic changes in the immune microenvironment of LCH bone metastasis and critical regulators are still unclear. Expression profiling by arrays of GSE16395, GSE35340, and GSE122476 was applied to detect the immune microenvironment changes in the development of LCH bone metastasis. The single-cell high-throughput sequencing of GSE133704, involved in LCH bone lesions, was analyzed. The online database Metascape and gene set variation analysis (GSVA) algorithms were used to detect the gene function of Gene Ontology (GO) and Kyoto Encyclopedia of Genes and Genomes (KEGG). The protein-protein interaction (PPI) network of hub regulators was constructed by the STRING database. In these results, key immune cells, such as Tem cells, NK T cells, CD8(+) T cells, and Th1 cells, were identified in LCH bone metastasis. These genes, which include LAG3, TSPAN5, LPAR5, VEGFA, CXCL16, CD74, and MARCKS, may significantly correlate with the cellular infiltration of B cells, aDCs, pDCs, cytotoxic cells, T cells, CD8+ T cells, T helper cells, and Tcm cells. In conclusion, our study constructed an atlas of the immune microenvironment of LCH bone metastasis. Genes including LAG3, TSPAN5, LPAR5, VEGFA, CXCL16, CD74, and MARCKS may be involved in the development of LCH bone metastasis. The hub gene-immune cell interactive map may be a potential prognostic biomarker for the progression of LCH bone metastasis and synergetic targets for immunotherapy in LCH patients.

Notch-dependent cooperativity between myeloid lineages promotes Langerhans cell histiocytosis pathology

Langerhans cell histiocytosis (LCH) is a potentially fatal neoplasm characterized by the aberrant differentiation of mononuclear phagocytes, driven by mitogen-activated protein kinase (MAPK) pathway activation. LCH cells may trigger destructive pathology yet remain in a precarious state finely balanced between apoptosis and survival, supported by a unique inflammatory milieu. The interactions that maintain this state are not well known and may offer targets for intervention. Here, we used single-cell RNA-seq and protein analysis to dissect LCH lesions, assessing LCH cell heterogeneity and comparing LCH cells with normal mononuclear phagocytes within lesions. We found LCH discriminatory signatures pointing to senescence and escape from tumor immune surveillance. We also uncovered two major lineages of LCH with DC2- and DC3/monocyte-like phenotypes and validated them in multiple pathological tissue sites by high-content imaging. Receptor-ligand analyses and lineage tracing in vitro revealed Notch-dependent cooperativity between DC2 and DC3/monocyte lineages during expression of the pathognomonic LCH program. Our results present a convergent dual origin model of LCH with MAPK pathway activation occurring before fate commitment to DC2 and DC3/monocyte lineages and Notch-dependent cooperativity between lineages driving the development of LCH cells.

Signaling pathways, microenvironment, and targeted treatments in Langerhans cell histiocytosis

Langerhans cell histiocytosis (LCH) is an inflammatory myeloid malignancy in the "L-group" histiocytosis. Mitogen-activated protein kinase (MAPK) pathway activating mutations are detectable in nearly all LCH lesions. However, the pathogenic roles of MAPK pathway activation in the development of histiocytosis are still elusive. This review will summarize research concerning the landscape and pathogenic roles of MAPK pathway mutations and related treatment opportunities in Langerhans cell histiocytosis. Video abstract.

Monitoring Circulating CD207+CD1a+ Cells in Langerhans Cell Histiocytosis and Clinical Implications

Langerhans cell histiocytosis (LCH) is a disorder characterized by an abnormal accumulation of CD207⁺ and CD1a⁺ cells in almost any tissue. Currently, there is a lack of prognostic markers to follow up patients and track disease reactivation or treatment response. Putative myeloid precursors CD207⁺ and CD1a⁺ cells were previously identified circulating in the blood. Therefore, we aim to develop a sensitive tracing method to monitor circulating CD207⁺ and CD1a⁺ cells in a drop of blood sample of patients with LCH. A total of 202 blood samples from patients with LCH and 23 controls were tested using flow cytometry. A standardized cellular score was defined by quantifying CD207⁺ and CD1a⁺ expression in monocytes and dendritic cells, based on CD11b, CD14, CD11c, and CD1c subpopulations, resulting in a unique value for each sample. The scoring system was validated by a receiver operating characteristic curve showing a reliable discriminatory capacity (area under the curve of 0.849) with a threshold value of 14, defining the presence of circulating CD207⁺ and CD1a⁺ cells. Interestingly, a fraction of patients with no evident clinical manifestation at the time of sampling also showed presence of these cells (29.6%). We also found a differential expression of CD207 and CD1a depending on the organ involvement, and a positive correlation between the cellular score and plasma inflammatory markers such as soluble CD40L, soluble IL-2Ra, and CXCL12. In conclusion, the analysis of circulating CD207 and CD1a cells in a small blood sample will allow setting a cellular score with minimal invasiveness, helping with prognostic accuracy, detecting early reactivation, and follow-up.

Neutralizing Anti-IL-17A Antibody Demonstrates Preclinical Activity Enhanced by Vinblastine in Langerhans Cell Histiocytosis

Langerhans cell histiocytosis (LCH) is an inflammatory myeloid neoplasm characterised by the accumulation into granulomas of apoptosis-resistant pathological dendritic cells (LCH-DCs). LCH outcome ranges from self-resolving to fatal. Having previously shown that, (i) monocyte-derived DCs (Mo-DCs) from LCH patients differentiate into abnormal and pro-inflammatory IL-17A-producing DCs, and (ii) recombinant IL-17A induces survival and chemoresistance of healthy Mo-DCs, we investigated the link between IL-17A and resistance to apoptosis of LCH-DCs. In LCH granulomas, we uncovered the strong expression of BCL2A1 (alias BFL1), an anti-apoptotic BCL2 family member. In vitro, intracellular IL-17A expression was correlated with BCL2A1 expression and survival of Mo-DCs from LCH patients. Based on the chemotherapeutic drugs routinely used as first or second line LCH therapy, we treated these cells with vinblastine, or cytarabine and cladribine. Our preclinical results indicate that high doses of these drugs decreased the expression of Mcl-1, the main anti-apoptotic BCL2 family member for myeloid cells, and killed Mo-DCs from LCH patients ex vivo, without affecting BCL2A1 expression. Conversely, neutralizing anti-IL-17A antibodies decreased BCL2A1 expression, the downregulation of which lowered the survival rate of Mo-DCs from LCH patients. Interestingly, the in vitro combination of low-dose vinblastine with neutralizing anti-IL-17A antibodies killed Mo-DCs from LCH patients. In conclusion, we show that BCL2A1 expression induced by IL-17A links the inflammatory environment to the unusual pro-survival gene activation in LCH-DCs. Finally, these preclinical data support that targeting both Mcl-1 and BCL2A1 with low-dose vinblastine and anti-IL-17A biotherapy may represent a synergistic combination for managing recurrent or severe forms of LCH.

High soluble HLA-DQB2 levels in posttransplant serum are associated with kidney graft dysfunction

HLA-DQB2 is a gene of limited polymorphism, with unknown function that presents at least two transcript variants: v1, which encodes the full-length beta-chain, and v2, which lacks exon 4 and could give rise to a soluble protein. We previously showed a strong correlation between high v2 expression in preimplantation biopsies (PIB) of kidneys from young (18- to 49-year olds) but not from old, deceased donors and 1-year posttransplant low (estimated glomerular filtration rate < 45 ml/min/1.73 m² ) graft function (GF). In this study, we aimed to investigate the impact of posttransplant soluble HLA-DQB2 (sDQB2) serum levels, v1 expression in PIB, and recipient HLA-DQB2 rs7453920 A/G polymorphism on GF. sDQB2 was evaluated by enzyme-linked immunosorbent assay in sera from 114 recipients, collected at least 1 year (median 2.1 years) after transplantation. Higher sDQB2 levels were observed in recipients of kidneys from young, but not from old, donors that had a ≥30% decline in GF within 1 year after blood collection for sDQB2 determination. Among the 15 recipients of kidneys from young donors with sDQB2 ≥ 1.52 ng/ml, 40% presented a ≥30% decline in GF, whereas this occurred in none of the 43 recipients with lower sDQB2 levels (p = 0.007; OR: 36.5). Expression of HLA-DQB2 variant 1, measured by reverse transcription-polymerase chain reaction (RT-PCR) in 92 PIB from young or old donors, did not significantly differ between transplants with high or low 4-year GF. HLA-DQB2 rs7453920 single nucleotide polymorphism (SNP) frequencies did not significantly differ between recipients with low or high 4-year GF. We conclude that HLA-DQB2 variant 1 expression in PIB and recipient rs7453920 SNP polymorphism are not associated with graft outcome. On the other hand, the association, in transplants of kidneys from young donors, between high posttransplant serum sDQB2 levels and decline in GF is a very interesting finding that deserves a validation study in a larger cohort.

Utility of Cyclin D1 Immunostaining in Cutaneous Xanthogranuloma

ABSTRACT

Mitogen-activated protein kinase (MAPK)/extracellular signal-regulated kinase pathway activation has recently been found to be associated with almost all Langerhans cell histiocytosis cases. In BRAF V600E mutation-positive cases, this activation is seen as a downstream activation. In addition, cyclin D1 is a downstream target of the MAPK pathway. Recent studies have argued in favor of using cyclin D1 as a potential neoplastic marker to differentiate Langerhans cell histiocytosis from other reactive Langerhans cell proliferations in the skin and lymph nodes. Therefore, we chose to study the immunohistochemical expression of cyclin D1 in cutaneous xanthogranuloma (XG) cases. Fifteen XG cases were retrieved and stained for cyclin D1, BRAF (v-raf murine sarcoma viral oncogene homolog B1), CD1a, and langerin (CD207). Twelve cases showed strong and diffuse nuclear positivity for cyclin D1, both in the XG cells and in the multinucleated osteoclast-like giant cells. Three cases showed focal weak nuclear staining for cyclin D1. All 15 cases showed negative immunoreactivity for BRAF, CD1a, and CD207. Although limited by small sample size, we conclude that most cases of cutaneous XG should show at least weak nuclear staining with cyclin D1. The histogenesis of XG is still largely unknown, and the finding of cyclin D1 positivity in a majority of cases may indicate a role for the MAPK/extracellular signal-regulated kinase pathway in cutaneous XG.

Mitogen-activating protein kinase pathway alterations in Langerhans cell histiocytosis

PURPOSE OF REVIEW

Langerhans cell histiocytosis (LCH) is a rare disorder characterized by the infiltration of involved tissues by specialized dendritic cells. The demonstration of the constant activation of the mitogen-activated protein kinase (MAPK) pathway in LCH lesions has been a breakthrough in the understanding of the pathogenesis of this rare disease. We will summarize the current knowledge on MAPK alterations in LCH and the new therapeutic options indicated by these findings.

RECENT FINDINGS

Since the description of the B-Raf proto-oncogene, serine/threonine kinase (BRAF)V600E mutation in LCH lesions, several other molecular alterations affecting the MAPK pathway have been identified in most cases. Based on these driver alterations, LCH cells were shown to be derived from hematopoietic precursors, which yielded the current concept of LCH as a myeloid inflammatory neoplasia. MAPK pathway inhibitors have emerged as an innovative therapy in severe forms of LCH, resulting in virtually no acquired resistance. However, although they are highly effective, their effect is only temporary, as the disease relapses upon discontinuation of the treatment.

SUMMARY

LCH is an inflammatory myeloid neoplastic disorder, driven by mutations activating the MAPK pathway. MAPK-targeted treatments represent an important stepforward in the management of patients with severe progressive LCH.

BRAFV600E-induced senescence drives Langerhans cell histiocytosis pathophysiology

Langerhans cell histiocytosis (LCH) is a potentially fatal condition characterized by granulomatous lesions with characteristic clonal mononuclear phagocytes (MNPs) harboring activating somatic mutations in mitogen-activated protein kinase (MAPK) pathway genes, most notably BRAFV600E. We recently discovered that the BRAFV600E mutation can also affect multipotent hematopoietic progenitor cells (HPCs) in multisystem LCH disease. How the BRAFV600E mutation in HPCs leads to LCH is not known. Here we show that enforced expression of the BRAFV600E mutation in early mouse and human multipotent HPCs induced a senescence program that led to HPC growth arrest, apoptosis resistance and a senescence-associated secretory phenotype (SASP). SASP, in turn, promoted HPC skewing toward the MNP lineage, leading to the accumulation of senescent MNPs in tissue and the formation of LCH lesions. Accordingly, elimination of senescent cells using INK-ATTAC transgenic mice, as well as pharmacologic blockade of SASP, improved LCH disease in mice. These results identify senescent cells as a new target for the treatment of LCH.

Expression of CD1 molecules and colony-stimulating factor 1 receptor in indeterminate cell histiocytosis

Indeterminate cell histiocytosis (ICH) and Langerhans cell histiocytosis (LCH) are rare histiocyte proliferating disorders with unknown etiologies. However, both tumor cells immunophenotypically share some features of Langerhans cells (LC), thereby expressing CD1a. Recent transcriptome analysis revealed that circulating CD1c⁺ myeloid dendritic cells are the potential precursor of LCH tumor cells. LC express CD1a as well as CD1c, but not CD1b. We discovered that both tumor cells express CD1c, but not CD1b, similar to LC. Moreover, like LC, both tumor cells express colony-stimulating factor 1 receptor (CSF-1R). Considering the crucial role of the interleukin (IL)-34/CSF-1R axis for the development and survival of LC, CSF-1R on both tumor cells might facilitate their survival and proliferation in situ. These data provide additional evidence to support the fact that ICH and LCH share immunophenotypical features with LC. In addition, we hypothesized that tumor cells in ICH and LCH survive and proliferate through IL-34-mediated CSF-1R signaling.

Bone marrow-derived myeloid progenitors as driver mutation carriers in high- and low-risk Langerhans cell histiocytosis

Langerhans cell histiocytosis (LCH) is a myeloid neoplasia, driven by sporadic activating mutations in the MAPK pathway. The misguided myeloid dendritic cell (DC) model proposes that high-risk, multisystem, risk-organ-positive (MS-RO+) LCH results from driver mutation in a bone marrow (BM)-resident multipotent hematopoietic progenitor, while low-risk, MS-RO- and single-system LCH would result from driver mutation in a circulating or tissue-resident, DC-committed precursor. We have examined the CD34+c-Kit+Flt3+ myeloid progenitor population as potential mutation carrier in all LCH disease manifestations. This population contains oligopotent progenitors of monocytes (Mo's)/macrophages (MΦs), osteoclasts (OCs), and DCs. CD34+c-Kit+Flt3+ cells from BM of MS-RO+ LCH patients produced Langerhans cell (LC)-like cells in vitro. Both LC-like and DC offspring from this progenitor carried the BRAF mutation, confirming their common origin. In both high- and low-risk LCH patients, CD34+c-Kit+Flt3+ progenitor frequency in blood was higher than in healthy donors. In one MS-RO+ LCH patient, CD34+c-Kit+Flt3+ cell frequency in blood and its BRAF-mutated offspring reported response to chemotherapy. CD34+c-Kit+Flt3+ progenitors from blood of both high- and low-risk LCH patients gave rise to DCs and LC-like cells in vitro, but the driver mutation was not easily detectable, likely due to low frequency of mutated progenitors. Mutant BRAF alleles were found in Mo's /MΦs, DCs, LC-like cells, and/or OC-like cells in lesions and/or Mo and DCs in blood of multiple low-risk patients. We therefore hypothesize that in both high- and low-risk LCH, the driver mutation is present in a BM-resident myeloid progenitor that can be mobilized to the blood.

Identification of Genes Encoding Antimicrobial Proteins in Langerhans Cells

Langerhans cells (LCs) reside in the epidermis where they are poised to mount an antimicrobial response against microbial pathogens invading from the outside environment. To elucidate potential pathways by which LCs contribute to host defense, we mined published LC transcriptomes deposited in GEO and the scientific literature for genes that participate in antimicrobial responses. Overall, we identified 31 genes in LCs that encode proteins that contribute to antimicrobial activity, ten of which were cross-validated in at least two separate experiments. Seven of these ten antimicrobial genes encode chemokines, CCL1, CCL17, CCL19, CCL2, CCL22, CXCL14 and CXCL2, which mediate both antimicrobial and inflammatory responses. Of these, CCL22 was detected in seven of nine transcriptomes and by PCR in cultured LCs. Overall, the antimicrobial genes identified in LCs encode proteins with broad antibacterial activity, including against Staphylococcus aureus, which is the leading cause of skin infections. Thus, this study illustrates that LCs, consistent with their anatomical location, are programmed to mount an antimicrobial response against invading pathogens in skin.

The triptych of mixed histiocytosis: a systematic review of 105 cases and proposed clinical classification

Histiocytoses are one of the ultimate diagnostic challenges that every physician face at least once in his/her life. Giving their protean manifestation and differentiated therapeutic needs, histiocytosis requires extensive characterization and multidisciplinary management. Mixed histiocytosis is an emerging group of syndromes defined by the overlap of Langerhans cell histiocytosis and another histiocytic disorder of different type. Despite rare, it may account for up to a fifth of systemic histiocytosis patients in some series. In this work, we comprehensively review for the first time the clinical, radiological, histopathological and molecular features of mixed histiocytosis in children and adults. Moreover, we propose a clinical classification in three groups that differentiate patients with systemic involvement and worse overall survival to other groups with more localized manifestations and indolent behavior, wanting to ease their recognition and treatment. Interestingly we also found that mixed histiocytosis harbor BRAFV600E mutations with a higher frequency comparing to all other histiocytoses, and may therefore benefit of specific inhibitory drugs.