Requirement of CD28 signaling in homeostasis/survival of TGF-beta converted CD4+CD25+ Tregs from thymic CD4+CD25- single positive T cells.
BACKGROUND
The thymus is a major organ that generates "natural" CD4+CD25+ T regulatory cells (Tregs). However, the detailed pathway(s) by which Tregs are developed remain a mystery. CD28-/- mice have profound decrease in Tregs, but the underlying molecular events remain largely undefined.


METHODS
CD4+CD25+ thymocytes from wildtype and CD28-/- mice were cultured with T-cell receptor (TCR) and transforming growth factor (TGF)-beta stimulation to generate CD25+ Tregs and their phenotype and function were studied in vitro and in vivo.


RESULTS
TGF-beta induced Foxp3 expression in thymic CD4+CD25+ cells and converted them to CD25+ Tregs. The converted Tregs expressed high levels of CD25, whereas the non-suppressive CD4+ T cells from the control cultures expressed CD25(low). CD28-/- thymic CD4+CD25+ cells showed transit lower levels of Foxp3 upon TCR and TGF-beta stimulation early in culture, but the defect in Foxp3 expression was restored to normal levels after 60-72 hr. Consequently, TGF-beta converted CD28-/- CD25+ cells to CD25+ Tregs that were indistinguishable from those of the wildtype mice. However, the total number of TGF-beta converted CD28-/- Tregs was significantly lower than that of wildtype mice. In vivo, TGF-beta converted CD28-/- CD25+ Tregs were less viable than those from the wildtype mice. Importantly, TGF-beta induced alloantigen specific CD4+CD25+ Tregs from thymic CD25-SP cells which also required CD28 to maintain their survival.


CONCLUSIONS
TGF-beta and TCR co-stimulation converts thymic CD4+CD25+ T cells into CD4+CD25+ Tregs by inducing Foxp3, and the contribution of CD28 stimulation to this process is mainly through maintaining survival of the induced Tregs.
Liu
Yongzhong
Y
Mucosal Immunology Unit, Oral Infection and Immunity Branch, National Institute of Dental and Craniofacial Research, National Institutes of Health, Bethesda, MD 20895, USA.
Amarnath
Shoba
S
Chen
WanJun
W
D019264:Adoptive Transfer; D000818:Animals; D018106:CD28 Antigens / Q000502:physiology*; D015496:CD4-Positive T-Lymphocytes / Q000187:drug effects / Q000276:immunology; D018929:Cell Culture Techniques; D002470:Cell Survival; D018920:Coculture Techniques; D017931:DNA Primers; D051858:Forkhead Transcription Factors / Q000235:genetics; D005786:Gene Expression Regulation / Q000187:drug effects / Q000276:immunology; D053645:Interleukin-2 Receptor alpha Subunit / Q000172:deficiency / Q000276:immunology; D051379:Mice; D008810:Mice, Inbred C57BL; D018345:Mice, Knockout; D020133:Reverse Transcriptase Polymerase Chain Reaction; D013154:Spleen / Q000276:immunology; D013601:T-Lymphocytes / Q000166:cytology / Q000187:drug effects / Q000276:immunology*; D013950:Thymus Gland / Q000276:immunology; D016212:Transforming Growth Factor beta / Q000494:pharmacology
D018106:CD28 Antigens; D017931:DNA Primers; D051858:Forkhead Transcription Factors; C498833:Foxp3 protein, mouse; D053645:Interleukin-2 Receptor alpha Subunit; D016212:Transforming Growth Factor beta