Expression of CD2 and CD25 on mast cell populations can be seen outside the setting of systemic mastocytosis.

BACKGROUND: Systemic mastocytosis (SM) is a diagnosis made using clinical, laboratory, and histologic parameters. Aberrant CD2 and/or CD25 expression on mast cells provides one minor criterion for a diagnosis of SM. To validate a tube (CD45/CD117/CD2/CD25) for mast cell evaluation, flow cytometry (FC) on residual material from marrow aspirates samples submitted to the hematopathology laboratory was performed.

METHODS: Samples evaluated (n = 98) had no clinical or morphologic suspicion for SM. Samples were excluded if there was history of a myeloid stem cell neoplasm. Ten documented cases of SM were evaluated for comparison.

RESULTS: Among cases without history of SM, 17.3% (n = 17) showed expression of CD2 and/or CD25 on ≥10% of the mast cell population (CD25 alone in 14 cases, CD2 alone in 2 cases, both in one case), while 82.6% (n = 81) showed no expression of these antigens. The percentage of mast cells showing aberrant CD2 and/or CD25 expression respectively ranged from 12.1% to 98.8% and 22.2% to 95.7% Interestingly, all of the cases with evidence of aberrant antigen expression on mast cells were collected post-therapy while 22.1% of the negative samples were collected pre-therapy. A cut-off of 60% CD25 expression on mast cells identified all cases of SM while minimizing false positives.

CONCLUSIONS: These findings demonstrate that aberrant expression of CD2 and/or CD25 may be seen on mast cells outside of the setting of SM. The data suggests that this phenomenon may be seen more commonly following chemotherapy and that FC of mast cells should be interpreted with caution in the post-chemotherapy setting. © 2015 International Clinical Cytometry Society.