We examined the flow cytometry findings in involved PB samples from 44 MF/SS patients, most of who had multiple sequential involved PB samples. Using a standard T-cell cytometry panel, we noted abnormal T-cell populations showing one or more antigen aberrancies in 93% of patients. In 5 MF patients, we noted multiple persistent phenotypically abnormal T-cell populations consistent with clonal heterogeneity. We present the first report on the stability of such abnormal immunophenotypes in the neoplastic T cells of SS/MF, over disease course. Despite varying treatments, immunophenotypic aberrancies noted were maintained over the course of disease in 95% of cases, excluding common variations in the level of CD7 expression. These findings support the hypothesis that these abnormal T-cell populations represent the neoplastic clone and provide strong support for the use of flow cytometry in routine monitoring of treatment response.
Tumor-associated variation in one or several pan-T cell antigens is a typical feature of neoplastic lymphocytes in many T-cell malignances and has been termed antigen "loss" or "deletion" [7
]. Using immunohistochemistry on a large bank of T-cell lymphomas, we have previously noted a frequency of such antigen deletion of 18% for CD2, 26% for CD3, 30% for CD5 and 68% for CD7 [8
]. Flow cytometric analysis can allow a more sensitive assessment of subtle variations in the levels of surface marker expression than immunohistochemistry [9
]. In non-neoplastic T-cell populations, we noted relatively narrow range of CD3, CD4 and CD5 expression with majority of cells clustering within one log intensity. In contrast, the aberrancies noted in expression in T-cell tumor often showed 2–3 log-fold intensity differences from the normal populations [5
Most routinely assessed T-cell diagnostic markers (e.g. TCR-α/β, CD3, CD4, CD8 and CD45) are components of the antigen receptor complex and are dynamically regulated during T-cell maturation and activation [13
]. In a subset of cases analyzed, we noted that surface CD3 levels frequently correlated with levels of TCR-α/β, but that CD3 did not always correlate with variations in the levels of surface CD4. We also found variations in MF/SS tumor cells in the expression level of CD45, a phosphatase that regulates TCR activation [14
]. The finding of frequent variations in the levels of these TCR-complex proteins in tumors is intriguing and suggests abnormal regulation or formation of the TCR complex in some tumor cells. This could be lead to dysregulated TCR signaling effecting growth regulation.
In previous studies, others and we have noted that MF/SS tumor cells frequently lacked CD26, and that this finding was helpful in identifying the tumor cell population [4
]. In this larger longitudinal series, we confirm these findings and noted that tumor cells showed a consistent pattern of CD26 loss over the course of disease. The stable loss of expression of CD26, a T-cell activation marker, in MF/SS suggests that this activation pathway may be nonfunctional in most MF/SS tumors. CD26 is an ectoenzyme, which delivers a costimulatory signal in T-cells; its expression is upregulated on T-cells in response to mitogenic signals and downregulated through interaction with the insulin-like growth factor II receptor [16
]. The consequence of CD26 loss is unclear; CD26 modulates the action of many molecules including the chemotactic activity of some chemokine important for T-cell migration to skin [17
In contrast, we noted that CD7 expression was frequently modulated in tumor cells over the disease course. Several previous studies have demonstrated that CD7 loss is not a predictable marker for detection of MF/SS cells, as its expression is variable in this tumor type [15
]. Although there is data to suggest that CD7-negative T-cells may represent a distinct differentiation pathway in normal and neoplastic helper T cells,[20
] the variable expression of CD7 in MF/SS that we have observed suggests that this marker may be highly modulated by tumor microenvironment and/or treatment.
Identification of a stable tumor-associated immunophenotype in MF/SS allows tumor cell quantification to assess response to therapy. Cluster analysis of abnormal flow cytometric immunophenotype is more reliable and easier to compare over time than morphologic analysis, and can detect PB involvement in a small number of cases without easily identifiable Sezary cells [12
]. Routine flow cytometric profiling is also not as technically demanding as molecular monitoring[22
] or electron microscopy. Such a separation of tumor from normal T-cells is also important in assessing the effects of therapies on normal leukocytes.
In conclusion, we found that the absence of CD26 and other aberrant surface expression of T cell-associated markers allowed identification of a discrete, quantifiable population in nearly all MF/SS patients with PB tumor cells. Immunophenotypic aberrancies are maintained consistently over the course of disease and can be used to monitor tumor response to therapy.