The combined histologic, molecular, and IHC studies of histiocyte–rich neoplasms described here provide important new insights into the origins and characteristics of spontaneous mouse HS and HAL while introducing a novel subset of these tumors: composites of HS and lymphoma. They also highlight the need for a phenotypic or molecular signature that defines transformed histiocytes. HS of humans is also of uncertain molecular pathogenesis.
In the absence of markers for transformed histiocytes, we suggest that HS is defined by germline configurations of immune receptor loci, the presence of myeloid markers, and the lack of expression of genes associated with other sarcoma types. The presence of abnormally large histioblasts is correlative but of uncertain significance. It is possible that IHC analyses for associated T and/or B cell lymphomas easily missed on examination of slides stained with HE could substitute for Southern blot hybridization analyses of immune receptor genes in diagnosing this malignancy.
By light microscopy, HS was shown to include cases with a predominance of round or spindle cells or mixed populations of the 2. Our phenotypic analyses provided strong evidence that the round cells represent a more mature population of histiocytes than that of the spindle cells, given the higher and lower levels of F4/80 expression, respectively. This perspective is based on the prior demonstration that expression of F4/80 increases with progressive maturation in the monocyte/macrophage lineage.35
Rare but highly informative cases from our series exhibited transitions from round to spindle cells in the same field in association with progressive reductions in expression of F4/80, a clear indication that the 2 histologic types were developmentally related in the same case. Analyses of other markers, including Mac–2 and lysozyme, confirmed a shared myeloid origin for the cytologically distinct populations of HS. Histopathology and immunophenotypic studies of a few cases in our series suggest that HS can originate from Kupffer cells in the liver as well as from macrophages in spleen or LN or from the uterus.
In addition to the categories of HS and HAL, we have tentatively identified a subset of histiocyte–rich tumors that are composites of HS and a lymphoid neoplasm. Support for this suggestion and distinction of the subset from HAL will require the identification and validation of molecular and/or IHC markers for transformed histiocytes. We also described another anomalous presentation, which included cases with histiocytes that appeared to express PAX5, suggesting lineage infidelity. This observation bears some similarities to rare human cases of clonally related follicular lymphomas and histiocytic/dendritic cell sarcomas.16
In these patients, both cell types had identical IgH gene rearrangements, and they shared a t(14;18) translocation involving IgH and BCL2. Similar clonal relationships have been observed between T cell lymphoblastic leukemia/lymphoma and Langerhans cell histiocytosis and between acute lymphoblastic leukemia and HS.17
Studies of cultured cell lines have long suggested a poorly understood plasticity between the B cell and myeloid lineages,4,11,29,32
and this concept has gained increasing support from a variety of in vivo studies.*
The features of murine HS described in this report have many similarities to those of true HS in humans. HS in humans is a rare neoplasm that is sometimes associated with lymphomas and is difficult to distinguish from DLBCL or anaplastic large cell lymphoma without IHC studies. The cells are usually large, with abundant eosinophilic cytoplasm and large round nuclei with vesiculated chromatin. Spindle cells can be observed but are usually focal, a contrast with the diffuse expansions of spindle–shaped cells seen in some of our cases. In humans, the tumors are accompanied by variable numbers of eosinophils, lymphocytes, and plasma cells. The histiocytic cells variably express lysozyme and a number of other histiocytic markers, such as CD68, S–100, and CD11c. Absence of Ig and TCR gene rearrangements is required for definition of this disease by some,9
although they can clearly be found in an extremely low proportion of cases.16
Mouse and human cases are clearly distinguished by levels of mitotic activity, with frequencies of Ki67–expressing cells in the 80% range being common in human cases and with 1% representing the extreme end for cases in mice. The etiology of HS in humans is not known, although recent studies indicated that genetic or epigenetic inactivation of PTEN, p16INK4A
, and p14ARF
may play a role in some cases of both species.5
HAL in the mouse has a number of features in common with T cell–rich/histiocyte-rich lymphomas in humans as well as a number of distinct differences,1,36
although human and mouse cases can be readily distinguished on histologic grounds from other cases of stroma–rich B cell lymphomas. Most human cases are marked by large numbers of polyclonal T cells and lesser populations of histiocytes and clonal B cells, whereas the B cell component is more readily appreciated in mouse HA. In humans, there is a subset of cases with histiocytic predominance that more closely resembles the histologic features in mice.1,18
Although human T cell–rich/histiocyte–rich B cell lymphomas are recognized as a subset of DLBCL,27
splenic cases associated with MZL have been described.13
In the mouse, MZL is the most common lymphoma type associated with cases of HAL and compound cases of HS; lymphomas of germinal center origin (FBL, CBL, and IBL) are a close second.
The HS described here bears a number of similarities to HS identified in several strains of genetically engineered mice, including those deficient in the cytochrome C p450 gene, as well as Cyp1b153
knockout mice infected with MuLV,37
with incidences in the range of 55 to 65%. The tumors were F4/80+ and were found in the spleen, liver, and sometimes the bone marrow. A screening for common proviral integration sites in tumors of the Cdkn2a
–deficient mice identified several genes associated with HS, including Ptpn6, Myst3, Dgke
, and Kif13a
In addition, HS developed in most mice infected with an acutely transforming Ha–Ras–containing retrovirus.19
Whether any of these genes are involved in the pathogenesis of HS occurring in NFS. V+ mice will be the subject of future studies.
In summary, the present study demonstrated 3 morphologically distinct types of HS, with round, spindle, and mixed populations of histiocytes. All 3 are related on the basis of gene expression and differentiation patterns, with mature round cells expressing higher levels of F4/80 than those of spindle cells. The combination of histology with assessment of Ig clonality by Southern blotting and staining for F4/80, Mac–2, and PAX5 provides great value for distinguishing HS from HAL and other histiocyte–associated neoplasms while suggesting composites of HS with lymphoma as a new diagnostic category. The presence in some tumors of PAX5–expressing histiocytes may also provide opportunities for understanding B myeloid–lineage plasticity in neoplasia.