CD1d structure and regulation on human thymocytes, peripheral blood T cells, B cells and monocytes

Human T cells expressing CD161 and an invariant T-cell receptor (TCR) alpha -chain (V alpha 24(invt) T cells) specifically recognize CD1d and appear to have immunoregulatory functions. However, the physiological target cells f or this T-cell population, and whether alterations in CD1d expression contr ibute to the regulation of V alpha 24(invt) T-cell responses, remain to be determined. A series of antibodies were generated to assess CD1d expression , structure and regulation on human lymphoid and myeloid cells. CD1d was ex pressed at high levels by human cortical thymocytes and immunoprecipitation analyses showed it to be a 48 000-MW glycosylated protein. However, after solubilization, the majority of the thymocyte CD1d protein, but not CD1d ex pressed by transfected cells, lost reactivity with monoclonal antibodies (m Abs) against native CD1d, indicating that it was alternatively processed. M oreover, thymocytes were not recognized by CD1d-reactive V alpha 24(invt) T -cell clones. Medullary thymocytes and resting peripheral blood T cells wer e CD1d(-), but low-level CD1d expression was induced on activated T cells. CD1d was expressed by B cells in peripheral blood and lymph node mantle zon es, but germinal centres were CD1d(-). Resting monocytes were CD1d(+) but, in contrast to CD1a, b and c, their surface expression of CD1d was not up-r egulated by granulocyte-macrophage colony-stimulating factor (GM-CSF) and i nterleukin-4 (IL-4) activation. These results demonstrate constitutive CD1d expression by human professional antigen-presenting cells and that post-tr anslational processing of CD1d may contribute to regulation of the activity of CD1d-specific T cells.