Adult T-Cell Leukemia/Lymphoma (ATLL)
The leukemia associated with four-leaf clover (flower) cells on peripheral blood smear is Adult T-cell Leukemia/Lymphoma (ATLL), caused by Human T-lymphotropic virus type 1 (HTLV-1) infection 1.
Characteristic Morphology
The hallmark "flower cells" in ATLL display:
- Markedly polylobated nuclei with deeply cleaved, petal-like or cloverleaf appearance
- Homogeneous and condensed chromatin
- Small or absent nucleoli
- Agranular, basophilic cytoplasm
- Multiple morphologic variations may be encountered 1
These atypical lymphocytes range from small bland-appearing cells to large bizarre forms with multilobulated nuclei, creating a polymorphous population on smear 2.
Diagnostic Confirmation
While flower cells are highly suggestive, HTLV-1 seropositivity is mandatory for ATLL diagnosis 1. The complete diagnostic workup requires:
- Peripheral blood smear: Lymphocytosis (absolute lymphocyte count >4000) with ≥5% abnormal T lymphocytes in acute/chronic subtypes 1
- Flow cytometry: CD4-positive mature T-cell phenotype expressing CD2, CD3, CD5, CD25, CD45RO, CD29, and HLA-DR; typically CD7-negative and CD26-negative 1
- HTLV-1 serology: Positive by ELISA and Western blot 1
- Molecular analysis: Monoclonal integration of HTLV-1 provirus (mandatory in atypical cases) 1
Important Caveats
Flower cells are not pathognomonic for ATLL. Recent case reports demonstrate that similar morphology can occur in:
However, the prevalence of true flower cells in HTLV-I positive individuals is only 6.8%, significantly higher than the 1.1% in seronegatives 5. This underscores that immunophenotyping and HTLV-1 serology are essential to distinguish ATLL from morphologic mimics.
Clinical Context
ATLL is endemic in Japan, the Caribbean, intertropical Africa, Brazil, Eastern Europe (Romania), and the Middle East (particularly Iran) 1. The aggressive acute and lymphoma subtypes carry very poor prognosis due to multidrug resistance, large tumor burden, hypercalcemia, and profound immunodeficiency 1.