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Summary CD1A is a cell surface protein expressed on Langerhans cells and cortical thymocytes that could potentially be used as an immunotherapeutic target in Langerhans Cell Histiocytosis (LCH), the cortical subtype of T‐cell acute lymphocytic leukaemia (T‐ALL) and other CD1A‐positive tumours. The monoclonal antibody (mAb) CR2113 was selected from a panel of six fully human mAbs isolated from a semi‐synthetic phage display library, based on specificity and avidity against cells expressing CD1 antigen variants. CR2113 recognized CD1A in T‐ALL cell lines and patient samples. Confocal microscopy revealed that the CR2113‐CD1A complex was internalized at 37°C. Furthermore, while CR2113 induced moderate complement‐dependent cytotoxicity (CDC), potent antibody‐dependent cell cytotoxicity (ADCC) activity was observed against CD1A expressing cell lines as well as T‐ALL cell lines and T‐ALL patient samples. In vivo experiments showed that CR2113 as a naked antibody has modest but specific anti‐tumour activity against CD1A‐expressing tumours. CR2113 is a high‐affinity human anti‐CD1A mAb with significant ADCC activity. These properties make CR2113 a candidate for clinical diagnostic imaging and therapeutic targeting of LCH as well as potential use in other clinical applications.

SummaryLangerhans cell histiocytosis (LCH) is a clonal, proliferative disorder of phenotypically immature CD1a + Langerhans cells (LC). The aetiology of LCH is unknown and data supporting an immune dysregulatory disorder as well as a clonal neoplasm have been reported. Telomere shortening has been associated with cancers and premalignant lesions as well as promoting chromosomal instability. To determine whether LCH LC have altered telomere lengths, we used dual detection of CD1a expression by immunofluorescence and telomere length by fluorescence in situ hybridization of LCH LC and lymphocytes in local, multisystem and systemic LCH and compared these with telomere lengths of LC and lymphocytes in reactive lymph nodes. LCH LC showed significantly shorter telomere lengths than LC from reactive lymph nodes or unaffected skin. Lymphocyte telomere lengths showed similar profiles among the different samples. These data show a significant telomere shortening in LCH LC in all stages of disease involvement compared with LC from reactive lymph nodes, suggesting that LCH may share mechanisms of telomere shortening and survival with clonal preneoplastic disorders and cancer, although an initiating infectious or immune event is still possible.

Neoplastic diseases of macrophages (M phi) and dendritic cells (DC), collectively called histiocytoses, are relatively rare. The etiology of most forms of histiocytosis is poorly understood, and the development of animal models is crucial for further research in this field. Previously, an animal model for malignant histiocytosis (MH), involving transformed histiocytic cells, has been generated by infecting mice with malignant histiocytosis sarcoma virus (MHSV). However, increased insight into the heterogeneity of M phi and DC, and the associated reappraisal of human proliferative diseases involving these cells inspired us to re-evaluate the mouse model. We analyzed spleen, bone marrow, and lymph nodes of susceptible mice at various time points after infection. From day 11 onwards, a heterogeneous population of cells, consisting of CD8 alpha(+) Langerin(+) DC, ER-MP58(+) CD11b(+) myeloid precursor cells, CD169(+) metallophilic M phi, and CD71(hi) erythroblasts, was affected by viral transformation. In different mice, these subsets expanded at different rates in different organs, causing a variable disease profile in terminal stages. Cell lines, which were generated from MHSV-transformed tumors, showed a DC-like morphology and phenotype, and appeared to be arrested in different stages of maturation. Upon injection into healthy mice, different preferential homing patterns were observed for the various cell lines, and the cells acquired distinct phenotypes depending on the organ of homing. This indicates that these transformed cells adapt to their microenvironment by switching between precursor, DC/Langerhans cell, and M phi phenotypes. Our results demonstrate that the MHSV model represents a heterogeneous neoplastic disease with characteristics of M phi/DC sarcomas.