Hydroa vacciniforme: a distinctive form of Epstein-Barr virus-associated T-cell lymphoproliferative disorders.

Hydroa vacciniforme (HV) is a cutaneous subset of Epstein-Barr virus (EBV)-associated T/NK lymphoproliferative disorders (LPDs). Our previous case series study clearly showed a clinical spectrum of EBV-associated T/NK LPDs including HV, hypersensitivity to mosquito bites (HMB), chronic active EBV infection (CAEBV), and hemophagocytic lymphohistiocytosis (HLH). Patients with HV are divided into two groups: a benign subtype designated "classic HV" (cHV) and more serious systemic HV (sHV), also called "HV-like LPD" in the 2017 World Health Organization (WHO) classification. Patients with cHV usually have an increased number of EBV-infected γδT cells and patients with sHV without HMB are further classified into two groups: γδT-cell- and αβT-cell-dominant types. Patients with HMB, with or without HV-like eruptions, have an increased number of EBV-infected NK cells in the blood. Patients with cHV and γδT-cell-dominant sHV show a favourable prognosis, but the other subtypes such as αβT-cell-dominant sHV and HMB have a poor prognosis with mortality rates of 11.5 and 3.51 per 100 person-years, respectively. In addition to the clinical subtypes and the dominant lymphocyte subsets, the poor prognostic indicators include onset age over nine years and expression of the reactivation marker, BZLF1 mRNA. No prognostic correlation has been reported for anti-EBV antibody titres or EBV DNA load. The clinical subtypes and their prognostic factors should be considered for therapeutic interventions.