A 62-year-old Indian man presents with a 18-month history of progressive erythema involving >85% of body surface area, severe pruritus, and generalized lymphadenopathy. Skin biopsy shows atypical lymphoid infiltrate with epidermotropism. Peripheral blood flow cytometry reveals the abnormal cells marked as **C** in the diagram — atypical CD4+ T-cells with cerebriform nuclei at a count of 1,200 cells/μL. Which of the following best characterizes the diagnostic and prognostic significance of these cells?