On CD4+ T lymphocytes CD28 functions as co-stimulatory molecule during antigen presentation by dendritic cells. But CD8+ cytotoxic cells apparently do not need CD28 for killing their targets..?
I've always thought that CD28, even on CD8 T cells is the second signal for activation. This doesn't have to mean killing, since CD8 T cells activation also includes expanison and cytokine production.
Interaction of CD28 on T lymphocyte with CD80 or CD86 on dendritic cell provide co-stimulatory signal necessary for survival of activated antigen-specific T lymphocytes and their successful expansion in immune response. Really, this interaction enhances immune response. The presence of CD28 on the surface of CD8 T cells charachterize CD8 subpopulations capable to further re-stimulation by antigen and subsequent expansion in secondary immune responses. The absence of CD28 on CD8 T cells indicates on terminally differentiated effector cytotoxic T lymphocytes (CTL), which die after performing of CTL function and can not proliferate.
Thank you Dmitry, for so good and thorough explanation! Now I started wondering: If human CD8+ T cells loose CD28, they acquire CD57. Is CD57 just a marker, or does it perform any function (receptor or something similar...)?
Dear Dainius, unlikely CD8+ T cells acquire CD57. CD57 is a marker for another way of lymphocyte differentiation - mature natural killer cells. I would recommend to you this review article. Article Functional Significance of CD57 Expression on Human NK Cells...
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