Serum‐induced basophil CD63 expression by means of a tricolour flow cytometric method for the in vitro diagnosis of chronic urticaria

Background:  Functional autoantibodies against the α ‐chain of the high‐affinity IgE receptor (Fc ɛ RI α ) identify a subset of patients with chronic urticaria (CU) due to autoreactivity, as assessed by an in vivo positive response to autologous serum skin test (ASST). We performed a study to standardize the serum‐induced basophil activation assay by flow cytometry (FCM) using a new tricolour method, assessing the diagnostic performance of this test in discriminating between ASST+ and ASST− CU patients. Methods:  Sera of 64 CU patients (22 ASST+ CU and 42 ASST− CU) and 10 healthy subjects were tested for their ability to induce basophil CD63 expression when incubated with whole blood of both atopic (D A ) and non‐atopic donors (D NA ). Using a triple‐labelled strategy with anti‐CD123, anti‐HLA‐DR and anti‐CD63 antibodies, CD63+ basophils were identified on a selected population of CD123+ HLA‐DR‐ cells. In 3 ASST+ CU patients who underwent cyclosporine therapy, the assay was performed before and after treatment. Results:  The ASST+ CU sera resulted in a significant higher induction of basophil CD63 expression compared with ASST− CU and healthy donors sera; when whole blood from D A was used, sensitivity and specificity of the assay were 95.5% and 90.5% respectively. ASST+ CU serum activity was significantly decreased during cyclosporine A treatment, in parallel with clinical remission. Conclusions:  Chronic urticaria serum‐induced CD63 expression assay performed on D A whole blood by means of our tricolour FCM method could be the most useful tool for identification of a subset of patients with autoimmune CU and may become a promising tool also for monitoring treatment efficacy.