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Q: T-Cells
I understand that there are 4 types of T-cells: killer, helper, suppresser, and... what is the fourth one and what does it do? (January, 2000)
(Administrator's Note:Though this is not a nutritional question per se, we thought it deserved an answer. So we sought the input of what I'll call a "Visiting Professor to the Virtual Faculty". Our thanks to George Carter for his response. George is the Director of Treatment Information Development at Direct AIDS Access Information Resources (DAAIR) as well as a treatment advocate for ACT UP New York. George has written extensively on treatment options, including nutritional and herbal ones, and they are available on the DAAIR Web Site. From the home page select "HIV Treatment Info Pages" and then "Articles" for a listing, including an excellent primer entitled "Evaluating Therapies I & II".
A: George Carter responds:
Hmmm...I don't think I buy the first part of the statement. Break it down into white blood cells and red blood cells. Under white blood cells, you have many lymphocytes, including T cells. They develop from hematopoietic progenitor cells into CD4+CD8+ and then, because they are processed in the thymus (wherefore T cells), they follow a course as either CD4-CD8+ or CD4+CD8-. These cells then are the naive cells. When exposed to antigen, they become "effector" cells. After the antigenic challenge subsides, most undergo apoptosis. Some remain as memory cells (debatably identified by either CD45RA {naive} or RO {memory}).
T cells can further be divided by whether the T-cell receptor is composed of an alpha/beta chain (most common) or gamma/delta (more in the mucosal regions). Most of these g/d cells are CD8+.
Natural killer cells are not processed in the thymus, I don't believe, so they're not technically T cells (although they're known as large granular lymphocytes). They're characterized by CD16+56+ expression. Going by a marker isn't accurate because macrophages can express CD4, but again, their maturation isn't thymus-dependent. B-cells, of course, are bone-marrow derived. There's also LAK cells.
Sometimes, T cells are defined by their function. CD8+ cells can be cytotoxic T lymphocytes (identifying infected cells and killing them) or suppressor cells that appear to dampen immune responses. But this issue is a bit murky, and some evidence exists for CD4 cells operating with a suppressor function. This may have more to do with cytokine expression patterns or idiotypic networks.
Finally, CD25 and CD38: CD25, the IL-2 receptor, is not a specific T cell marker, but more one of activation marker found also on macrophages and B cells. CD38 is similarly found on thymocytes, activated T cells and B cells. It's an activation marker (like HLA-DR) often found on CD8 cells--and indeed, Giorgi's work suggests this can be a part of the immunopathogenesis. (Others, like Lubaki et al., found CD8+CD38-DR-CTLs were more effective in keeping HIV load lower; J AIDS, 1999;22(1):19-30).
