Differentiation of monoclonal B lymphocytosis of undetermined significance (MLUS) and chronic lymphocytic leukemia (CLL) with weak CD5 expression from CD5 − CLL

Chronic lymphocytic leukemia (CLL) is recognized as a unique lymphoproliferative disorder of CD5 + B cells. However, many published series of CLL included a subgroup of CD5 − cases. CD5 − B cell CLL is a topic of controversy and its nature and true incidence remain unclear. We in this study performed a retrospective analysis of a total of 128 consecutive patients with a diagnosis of CLL and available immunophenotypic record. Of these, 14 cases were previously considered CD5 − CLL. From a further analysis of clinical, hematological and immunophenotypic results, we have reclassified seven of the patients as having weak or dim expression of CD5 and four patients as being monoclonal B lymphocytosis of undetermined significance (MLUS). The remaining three cases had clinical and morphological features consistent with prolymphocytic leukemia (PLL) or mixed CLL/PLL. Our results suggest that the CD5 − phenotype probably does not qualify for CLL. Previous CD5 − CLL may include false negatives due to heterogeneity of the intensity of CD5 expression, CD5 − MLUS and variant CLL; the latter likely represents CLL in transformation. All the patients with MLUS were found to have a mild and non-progressing lymphocytosis with CD5 − phenotype. These features may be used to differentiate them from CLL.