Peripheral human CD8+CD28+T lymphocytes give rise to CD28–progeny, but IL-4 prevents loss of CD28 expression

At birth, virtually all peripheral CD8+ T cells express the CD28 co-stimulatory molecule, but healthy human adults accumulate CD28–CD8+ T cells that often express the CD57 marker. While these CD28– subpopulations are known to exert effector-type functions, the generation, maintenance and regulation of CD28– (CD57+ or CD57–) subpopulations remain unresolved. Here, we compared the differentiation of CD8+CD28brightCD57– T cells purified from healthy adults or neonates and propagated in IL-2, alone or with IL-4. With IL-2 alone, CD8+CD28brightCD57– T cell cultures yielded a prevailing CD28– subpopulation. The few persisting CD28dim and the major CD28– cells were characterized by similar telomere shortening at the plateau phase of cell growth. Cultures from adults donors generated four final CD8+ phenotypes: a major CD28–CD57+, and three minor CD28–CD57–, CD28dimCD57– and CD28dimCD57dim. These four end-stage CD8+ subpopulations displayed a fairly similar representation of TCR Vβ genes. In cultures initiated with umbilical cord blood, virtually all the original CD8+CD28bright T cells lost expression of CD28, but none acquired CD57 with IL-2 alone. IL-4 impacted on the differentiation pathways of the CD8+CD28brightCD57– T cells: the addition of IL-4 led both the neonatal and the adult lymphocytes to keep their expression of CD28. Thus, CD8+CD28brightCD57– T cells can give rise to four end-stage subpopulations, the balance of which is controlled by both the cytokine environment, IL-4 in particular, and the proportions of naive and memory CD8+CD28+ T cells.