Pityriasis lichenoides chronica: stratification by molecular and phenotypic profile

Pityriasis lichenoides (PL) has traditionally been classified as a benign papulosquamous disease. However, there is an increasing literature precedent that suggests that PL should instead be considered a form of cutaneous lymphoid dyscrasia. We prospectively encountered 46 patients with a diagnosis of PL and used immunohistochemical and multiplex polymerase chain reaction fragment size analysis to assess for phenotypic abnormalities and for T-cell clonal restriction, respectively. We categorized them into 2 groups based on the molecular profile, namely, those cases that showed a monoclonal and/or a restricted oligoclonal profile versus those cases that were polyclonal. Half of all the cases studied showed a monoclonal and/or an oligoclonal restricted T-cell repertoire. From a clinical perspective, 2 cases in this group manifested skin lesions compatible with mycosis fungoides (MF). All of the other cases demonstrated a persistent but nonprogressive clinical course characterized by periods of regression and recurrence. In any case in which there were multiple biopsies, the same T-cell dominant clonotypes, be it in the context of representing a true monoclonal and/or oligoclonal pattern, were implicated over time and at different biopsy sites, including 2 cases in which there was a subsequent evolution to MF. Substantial losses of CD7 and CD62L were seen in both monoclonal/oligoclonal and polyclonal cases of PL, although both values of percentage reduction were greater in the monoclonal/oligoclonal cases. A dominance of CD8 lymphocytes was seen in more than half of all the cases of PL and held to be reactive in nature, potentially directed against clonally restricted CD4 cells. CD4/CD25+ (Foxp3+) T cells averaged 24% in the polyclonal cases; it was 12% in the monoclonal variants of PL. We conclude that PL is a form of indolent cutaneous T-cell dyscrasia. The limited propensity for progression to MF may reflect internal countercheck mechanisms of controlling clonally restricted CD4+ T-cell proliferations via CD8 and CD4/CD25+ regulatory T cells.