Do you know of any peripheral blood mononuclear cells that would express any amount (beit low or high) of CD3 on their surface (other than T-cells)?
CD3 is initially expressed in the cytoplasm of pro-thymocytes, the stem cells from which T-cells arise in the thymus. The pro-thymocytes differentiate into common thymocytes, and then into medullary thymocytes, and it is at this latter stage that CD3 antigen begins to migrate to the cell membrane. The antigen is found bound to the membranes of all mature T cells, and in virtually no other cell type, although it does appear to be present in small amounts in Purkinje cells. This high specificity, combined with the presence of CD3 at all stages of T cell development, makes it a useful immunohistochemical marker for T cells in tissue sections. The antigen remains present in almost all T-cell lymphomas and leukaemias, and can therefore be used to distinguish them from superficially similar B-cell and myeloid neoplasms.
As you know CD3 is
a protein complex and is composed of four distinct chains. In mammals, the complex contains a CD3γ chain, a CD3δ chain, and two CD3ε chains.
Also on a side note, there are several other non peripheral mononucleated cells besides Purkinje cells that can express CD3, but mostly in pathological circumstances. For instance Warthin-Finkeldey cells, also known as polykaryocyes, display CD3, although the origin of these cells is not certain (they might be multinucleated T-cells).
(PS: If you want more information on CD3, I found this site very interesting.)