In recent years histiocytic or dendritic cell neoplasms clonally related to low grade B-cell lymphomas and leukemias have been described.7–8, 14
This phenomenon has been attributed to lineage plasticity of the underlying B-cell neoplasm, a phenomenon first recognized in murine systems.21–22
Prior to experimental data clarifying the nature of these events, there had been sporadic case reports of histiocytic or dendritic cell tumors associated with follicular lymphoma, CLL/SLL, and marginal zone lymphomas.10, 12–13
Feldman et al. were the first to provide molecular genetic evidence of a common clonal origin for follicular lymphoma and histiocytic/dendritic sarcoma in 8 cases, in which it was shown that the follicular lymphomas and histiocytic/dendritic sarcomas each shared a common IGH@ gene rearrangement and also carried the BCL2/IGH@ translocation, as shown by FISH.7
Thus far, a clonal relationship between CLL/SLL and a interdigitating dendritic cell sarcoma was only demonstrated in a single case report,14
and clinical or molecular risk factors for this occurrence have not been explored.
In this study, we report a series of 7 patients with both CLL/SLL and a neoplasm with properties of interdigitating dendritic cells, Langerhans cells, or histiocytes. All patients were elderly males. The CLL/SLL preceded the development of the histiocytic or dendritic sarcoma in 6 patients, and was synchronous with the secondary neoplasm in one patient. In all cases the CLL/SLL and histiocytic/dendritic cells existed in different proportions in the same specimen. A clonal relationship between the CLL/SLL and the histiocytic/dendritic cell tumors was demonstrated by identical clonal IGH@ or IGK@ gene rearrangements in all 7 patients, with the sequence identity of the corresponding V-D-J junctions confirmed in 5 patients. FISH analysis demonstrated chromosome 17p deletion or aneusomy in the histiocytic or dendritic cell sarcomas in 5 patients, and 17p deletion in the corresponding CLL/SLL in 2 patients.
In prior reports the tumors associated with CLL/SLL were all interpreted as interdigitating dendritic cell sarcomas.10, 13–14, 23–24
Our experience is somewhat similar in that 6 of 7 cases in our series expressed S100, and showed some evidence of dendritic cell differentiation. One of our cases expressed both CD1a and langerin, and thus, showed evidence of Langerhans cell differentiation. Transdifferentiation to dendritic cells rather than macrophages may be a preferred pathway, since dendritic cells can derive from either the common myeloid or lymphoid precursor.25
One case had a very immature phenotype, lacking convincing evidence of mature histiocytes or dendritic cells, and one case, while being focally S100+, expressed both CD163 and CD68, resembling more mature macrophages. Thus, while an interdigitating dendritic cell origin is most often seen, the secondary neoplasms can exhibit a wide spectrum of histiocytic and dendritic cell features.
A common clonal origin identified by immunoglobulin gene PCR provided strong evidence of transdifferentiation from CLL/SLL to histiocytic/dendritic cell sarcoma. Of note, sequence analysis showed preferential usage of the IGHV4-39 gene segment in the V-D-J rearrangements of the CLL/SLL and corresponding tumors. IGHV4-39 has been shown to be an independent risk factor for CLL transformation to diffuse large B-cell lymphoma.26
Thus, the use of this gene segment may be reflective of genetic instability in the B-cell clone. By FISH, all but one case of histiocytic/dendritic sarcoma had chromosomal abnormalities that were absent in the corresponding CLL/SLL, whereas no case of CLL/SLL had chromosomal abnormalities not observed in the other tumor. Thus, the FISH studies also provided indirect evidence that the development of the histiocytic/dendritic cell sarcomas was a secondary event. It would be of interest to know if the underlying CLL/SLL showed evidence of somatic hypermutation of the IGH@ genes. Unfortunately, due to the fragmented nature of the DNA obtained from the paraffin embedded tissue, analysis of VH gene mutations could not be performed.
The events that promote the transdifferentiation of a mature B-cell to a histiocyte or interdigitating dendritic cell are not well-understood. However, recent data have proven the potential for continued lineage plasticity in fully differentiated cells. For example, as few as 4 transcription factors (Oct4, Sox2, c-Myc, and Klf4) can reprogram terminally differentiated fibroblasts and hematopoietic precursor cells, including pro-B and Pre-B cells, to pluripotent stem cells. 27,28,29,30–31
In the hematopoietic system, deletion of the master regulator of B-cell development, PAX5, in mature B-cells led to de-differentiation into uncommitted progenitor cells and subsequent differentiation along the T-cell lineage.32–33
Xie et al showed that enforced expression of transcription factors C/EBPα and C/EBPβ led to direct transdifferentiation of mature B-cells to macrophages.21
These experiments, although all artificial with ectopic expression of higher than physiologic levels of transcription factors, suggested that lineage plasticity of mature cells can be achieved by changes of a few important transcription factors.
Two major hypotheses have been proposed for the molecular transformation of B-cell lymphoma to histiocytic or dendritic cell neoplasms. One is the direct transdifferentiation of neoplastic B-cells into malignant histiocytes or dendritic cells. The other involves a two-step process of transformation with first de-differentiation of neoplastic B-cells to early progenitors and subsequent re-differentiation along the histiocytic/dendritic lineage. The latter hypothesis would require the identification of an intermediate stage that could be linked genetically to the neoplastic B-cells. To date, intermediate stem cell-like forms have not been identified. However, one case in our series might represent such an intermediate step. The tumor in case 4 had a very immature phenotype, expressing CD4 and PU.1, but lacking more mature markers such as CD163, lysozyme, and S100.
A third theoretical possibility would be the presence of a common neoplastic progenitor with differentiation along both B-cell and histiocytic/dendritic lineages at different times, as can be seen in chronic myelogenous leukemia. Similar to leukemic stem cells,1
such neoplastic progenitors may be mitotically inactive and resistant to chemotherapy. Dendritic cells can arise from both the common myeloid and common lymphoid progenitors.25
However, a stem cell population was not identified in our cases, making such a scenario unlikely. A common genetic marker, such as BCR/ABL
, would be required to link all three stages of differentiation.
The process of differentiation is controlled by transcription factors. The B-cell fate is determined by only a few transcription factors, including E2A, EBF, PAX5 and PU.1.34–35
Of these transcription factors, PAX5 is the single most important one for lineage commitment and maintenance of B-cell identity and it functions throughout the life of B-cells, until terminal differentiation to plasma cells.33, 36
At the final stage of B-cell differentiation, PAX5 is suppressed by Blimp1, leading to loss of B-cell identity, an event that facilitates the upregulation of genes specific for plasma cells.37–38
As a result, mature B-cells have inherent plasticity for lineage commitment during terminal differentiation to plasma cells.39
The myeloid cell fate is determined by PU.1, CEBPα and CEBPβ.40
PU.1 is involved in early multilineage differentiation and the expression levels are different in subsequent lineages.41
We tested the expression of some of the transcription factors involved in B-cell and myeloid differentiation. In all 7 histiocytic or dendritic cell neoplasms, PU.1 was highly expressed, while PAX5 was completely negative in 5 cases. Interestingly, variable and patchy PAX5 expression was detected in the dendritic cell tumors in two cases. The presence of this restricted B-cell marker is consistent with the “transdifferentiation” model, as incompletely transdifferentiated cells with hybrid phenotype may be identified.
The expression of CEBPβ was variable and weak in all but one case from our series, in contrast to the strong staining in histiocytic/dendritic sarcomas reported by Feldman et al.7
The weak expression was confirmed by contrasting the tumor cells with background histiocytes showing strong CEBPβ staining in the same sections. Interestingly, the one case with strong expression of CEBPβ was case 7, which showed evidence of histiocytic rather than dendritic cell features, and was more similar to the histiocytic tumors reported by Feldman et al.
Fraser et al. studied the expression of PU.1, CEBPα, and CEBPβ by RT-PCR in a case of CLL/SLL with subsequent interdigitating dendritic cell sarcoma (also case 5 in our series).14
Interestingly, they showed high levels of CEBPβ by RT-PCR in both the CLL/SLL cells and the dendritic cell tumor. CEBPβ was not detected by immunohistochemistry in any case of CLL/SLL in our series, including case 5. The reasons for discrepancy in the results by RT-PCR vs. immunohistochemistry are not clear, but perhaps contamination of myeloid cells in the bone marrow sample involved by CLL/SLL contributed to the RT-PCR results.
Iwama et al. showed that enforced PU.1 expression and impaired CEBP function promoted Langerhans cell differentiation from myeloid progenitor cells.42
While the interaction of CEBP and PU.1 in interdigitating dendritic cell differentiation is not well established, the heterogeneous expression of CEBPβ coupled with the high level of PU.1 is consistent with the high incidence of tumors with Langerhans cell and interdigitating dendritic cell differentiation in our series.
The down-regulation of PAX5
is crucial for loss of B-cell identity.33
Somatic mutations of PAX5 have been described in diffuse large B-cell lymphomas.43
However, in a recent study, no somatic mutations of PAX5
were identified in two cases of histiocytic sarcoma transformed from B-cell lymphoma.44
Due to the limited quantity of microdissected DNA from paraffin embedded tissue, we could not perform genomic PCR to assess somatic mutations of PAX5
in the current study. Inhibition of PAX5
may also be caused by chromosomal changes. However, changes in chromosome 9p13 (PAX5
gene locus) were not identified by comparative genomic hybridization by Fraser et al.14
Given that PAX5
can be suppressed by multiple transcription factors, including CEBPβ. it is likely that the initial genetic event leading to transdifferentiation of CLL/SLL involves genes other than PAX5
In our series, the most common cytogenetic changes associated with CLL, such as deletion 13q, deletion 11q, deletion 6q, and trisomy 12 were not present in the CLL/SLL cells by FISH in the 6 cases studied. Fraser et al. reported trisomy 12 in both the CLL/SLL and interdigitating dendritic cell sarcoma in their case, representing case 5 in our series.14
In contrast, there were two cases of CLL/SLL with 17p13 deletion, an incidence (33%) higher than expected for de novo CLL/SLL. 17p abnormalities were also very common in the histiocytic/dendritic sarcomas, identified in 5/6 cases studied (83%). These findings suggest that chromosome 17p abnormalities may be a potential risk factor for transformation of CLL/SLL. 17p deletions are uncommon in de novo CLL/SLL, but when detected are associated with a more aggressive clinical course.45–46
The two cases with 17p13 deletion in the CLL/SLL component cells (cases 3 and 7) were stained for p53. TP53 was over expressed in the histiocytic/dendritic tumors in both cases, but in the CLL/SLL cells only in one case.
In summary, we extend the spectrum of histiocytic and dendritic cell sarcomas that may be associated with B-cell neoplasms. In CLL/SLL most secondary neoplasms show evidence of interdigitating dendritic cell differentiation, with only rare tumors composed of cells with features of mature histiocytes or macrophages. Larger series with special emphasis on the clinical course and treatment response are needed to better understand the response to different treatment strategies.