Histopathologic, Immunologic, and Cytogenetic Characteristics of LCH
Cell of Origin and Biologic Correlates
Cytogenetic and Genomic Studies
Cytokine Analysis by Immunohistochemical Staining and Gene Expression Array Studies
Human Leukocyte Antigen (HLA) Type and Association With LCH
Cell of Origin and Biologic Correlates
Modern classification of the histiocytic diseases divides them into dendritic cell–related, monocyte/macrophage-related, or true malignancies. Langerhans cell histiocytosis (LCH) is a dendritic cell disease.[1,2] The Langerhans cells (LCH cells) in LCH lesions are immature cells that express the monocyte marker CD14, which is not found on normal skin Langerhans cells (LCs). Comprehensive gene expression array data analysis on LCH cells is consistent with the concept that the skin LC is not the cell of origin for LCH. Rather it is likely to be a myeloid dendritic cell, which expresses the same antigens (CD1a and CD207) as the skin LC. This concept was further supported by a study reporting that the transcription profile of LCH cells was distinct from myeloid and plasmacytoid dendritic cells, as well as epidermal LCs.
LCH lesions also contain lymphocytes, macrophages, neutrophils, eosinophils, fibroblasts, and sometimes multinucleated giant cells. In the brain, the following three types of histopathologic findings have been described in LCH:
- Mass lesions in meninges or choroid plexus with CD1a-positive LCH cells and predominantly CD8-positive lymphocytes.
- Mass lesions in connective tissue spaces with CD1a-positive LCH cells and predominantly CD8-positive lymphocytes causing an inflammatory response and neuronal loss.
- Predominantly CD8+ lymphocyte infiltration with neuronal degeneration, microglial activation, and gliosis.
Normally, the LC is a primary presenter of antigen to naïve T-lymphocytes. However, in LCH, the LCH cell does not efficiently stimulate primary T-lymphocyte responses. Antibody staining for the dendritic cell markers, CD80, CD86, and class II antigens, has been used to show that in LCH, the abnormal cells are immature dendritic cells that present antigen poorly and are proliferating at a low rate.[3,7,8] Transforming growth factor-beta (TGF-beta) and interleukin (IL)-10 are possibly responsible for preventing LCH cell maturation in LCH. The expansion of regulatory T cells in LCH patients has been reported. The population of CD4-positive CD25(high) FoxP3(high) cells was reported to comprise 20% of T cells and appeared to be in contact with LCH cells in the lesions. These T cells were present in higher numbers in the peripheral blood of LCH patients than in controls and returned to a normal level when patients were in remission.Etiology Cytogenetic and Genomic Studies
Studies showing clonality in LCH using polymorphisms of methylation-specific restriction enzyme sites on the X-chromosome regions coding for the human androgen receptor, DXS255, PGK, and HPRT were published in 1994.[12,13] Biopsies of lesions with single-system or multisystem disease were found to have a proliferation of LCH cells from a single clone. Pulmonary LCH in adults is usually nonclonal. Cytogenetic abnormalities in LCH have rarely been reported. One study described an abnormal clone t(7;12)(q11.2;p13) from a vertebral lesion of one patient. This study also reported nonclonal karyotypic abnormalities in three patients. An increase in chromosomal breakage was also noted.
Comparative genomic hybridization has been used to analyze bone and pulmonary LCH cells with conflicting results.[14,16-18] Thus, there is some doubt if comparative genomic hybridization can reliably identify mutations in LCH.
One report has shown significantly shortened telomeres in lesional LCH cells compared with LCs in inflammatory disorders such as dermatopathic lymphadenitis. However, another group found telomere length of LCH cells from skin multisystem lesions were long compared with those from bone lesions that were heterogeneous in length. Telomerase was more often expressed in skin LCH lesions than in bone lesions. In another study evaluation of peripheral blood leukocyte DNA from high-risk LCH patients showed polymorphisms of two cytokine genes (IL-4 and interferon gamma), which were associated with high-expressor phenotypes.
Activating mutation of the BRAF gene (V600E) was detected in 35 of 61 (57%) LCH biopsy samples, with mutations being more common in patients younger than 10 years (76%) than in patients aged 10 years and older (44%). This was confirmed by a group that tested flow-sorted CD1a cells from fresh lesions and found 10 of 16 samples had a pathogenic BRAF mutation. Nine cases had the BRAF V600E mutation, and one additional case had a novel mutation, BRAF 600 DLAT, which demonstrated upregulation of ERK. These authors could not identify any clinical characteristics associated with the BRAF mutant genotype, even when they added their population (N = 16) to those previously reported (N = 61). A study of pulmonary lesions from five adults with lung LCH using a next-generation sequencing method to identify mutational hot spots in 46 cancer genes found two of five patients had the BRAF V600E mutation in all nodules tested.
Activating BRAF mutations are also found in selected nonmalignant conditions (e.g., benign nevi)  and low-grade malignancies (e.g., pilocytic astrocytoma).[26,27] All of these conditions have a generally indolent course with spontaneous resolution sometimes occurring. This distinctive clinical course may be a manifestation of oncogene-induced senescence.[25,28]Cytokine Analysis by Immunohistochemical Staining and Gene Expression Array Studies
Immunohistochemical staining of LCH lesions have shown apparent upregulation of the chemokines CCR6 and possibly CCR7.[29,30] In an analysis of gene expression in LCH by gene array techniques, 2,000 differentially expressed genes were identified. Of 65 genes previously reported to be associated with LCH, only 11 were found to be upregulated in the array results. The most highly upregulated gene in both CD207 and CD3-positive cells was osteopontin; other genes that activate and recruit T cells to sites of inflammation are also upregulated. The expression profile of the T cells was that of an activated regulatory T-cell phenotype with increased expression of FOXP3, CTLA4, and osteopontin. These findings support a previous report on the expansion of regulatory T cells in LCH. There was pronounced expression of genes associated with early myeloid progenitors including CD33 and CD44, which is consistent with an earlier report of elevated myeloid dendritic cells in the blood of LCH patients. A model of "Misguided Myeloid Dendritic Cell Precursors" has been proposed whereby myeloid dendritic cell precursors are recruited to sites of LCH by an unknown mechanism and the dendritic cells in turn recruit lymphocytes by excretion of osteopontin, neuropilin-1, and vannin-1.
Several investigators have published studies evaluating the level of various cytokines or growth factors in the blood of patients with LCH that have included many of the genes found not to be upregulated by the gene expression results discussed above. One explanation for elevated levels of these proteins is a systemic inflammatory response with the cytokines/growth factors being produced by cells outside the LCH lesions. A second possible explanation is that macrophages in the LCH lesions produce the cytokines measured in the blood or are concentrated in lesions.
IL-1 beta and prostaglandin GE2 levels were measured in the saliva of patients with oral LCH lesions or multisystem high-risk patients with and without oral lesions; levels of both were higher in patients with active disease and decreased after successful therapy.Human Leukocyte Antigen (HLA) Type and Association With LCH
Specific associations of LCH with distinct HLA types and extent of disease have been reported. In a study of 84 Nordic patients, those with only skin or bone involvement more frequently had HLA-DRB1*03 type than those with multisystem disease. In 29 patients and 37 family members in the United States, the Cw7 and DR4 types were significantly more prevalent in Caucasians with single-bone lesions.References
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