Efalizumab (anti-CD11a)-induced increase in peripheral blood leukocytes in psoriasis patients is preferentially mediated by altered trafficking of memory CD8+ T cells into lesional skin

Therapeutic administration of efalizumab, a humanized antibody to CD11a, induces a marked but reversible increase of peripheral lymphocytes in psoriasis patients. In this study, 13 patients were treated with 12 weekly subcutaneous doses (2 mg/kg/week) of efalizumab, and all 13 patients had increases in leukocyte counts. This increased white blood cell count was mainly due to a 3- to 4-fold increase in the number of circulating CD3⁺ lymphocytes during active treatment. Both naive and memory populations of CD4⁺ and CD8⁺ lymphocytes in the peripheral blood increased, with the largest increase observed in memory CD8⁺ T cells. This CD8 ⁺ memory T cell subset is a prominent T cell population found in psoriatic skin. An increase in Type 1 (IFN-γ producing) T cells was also observed during treatment. Both components of LFA-1, CD11a and CD18, were downregulated during treatment, and surprisingly the integrins CD11b and β7 were similarly reduced. We conclude that efalizumab most likely blocks cutaneous entry of memory CD8⁺ T cells, a highly disease-relevant cell population. The relatively smaller increase in naive peripheral blood T cells could be attributed to reduced trafficking of naive T cells.