Following lymphocyte depletion, homeostatic mechanisms drive the reconstitution of lymphocytes. We prospectively studied this process in 16 patients for 1 year after a single pulse of treatment with Campath‐1H, a humanised anti‐CD52 monoclonal antibody. We observed two phases of lymphocyte reconstitution. In the first 6 months after treatment the precursor frequency and proliferation index of the patients’ autologous mixed lymphocyte reaction increased; the depleted T cell pool was dominated by memory T cells, especially CD4⁺CD25^high T cells, a putative regulatory phenotype; and there was a non‐significant rise in peripheral mononuclear cell FoxP3 mRNA expression and fall in constitutive cytokine mRNA expression. In the later phase, from 6‐to‐12 months after Campath‐1H, these changes reversed and there was a rise in ROG mRNA expression. However, total CD4⁺ numbers remained below 50% of pre‐treatment levels at 12 months, perhaps reflecting a failure in homeostasis. This was not due to an impaired IL‐7 response, as in rheumatoid arthritis, nor to a lack of IL‐7 receptors, which are found on fewer human CD4⁺CD25^high than naive cells. We speculate that CCL21 and IL‐15 responses to lymphopaenia may be suboptimal in multiple sclerosis.

See accompanying commentary: http://dx.doi.org/10.1002/eji.200535385