Most immunologists are, by now, familiar with the population of naturally occurring regulatory T cells identified as CD4+CD25+ T cells. However, under some circumstances, CD4+CD25- T cells also display regulatory activity in vivo.

We have reported that the development of spontaneous EAE in MBP-specific T cell receptor (TCR) transgenic mice can be efficiently controlled by CD4+ T cells depleted of CD25+ T cells (1, 2). Using a TCR transgenic model of spontaneous diabetes, Gonzalez et al. showed a similar dose-response protective effect of CD4+CD25+ and CD4+CD25- T cells (3). Using a TCR transgenic model in which the antigen recognized by the transgenic TCR was also expressed as a transgene, Apostolou et al. showed that the anergic CD4+CD25+ and CD4+CD25- T cells had nearly identical capacity to block naïve T cell proliferation in vitro and in vivo (4). Moreover, we have shown that TCR transgenic-immunoglobulin knocked-in mice, which develop hyper IgE response upon immunization, can be protected from the hyper IgE response by either CD4+CD25+ or CD4+CD25- T cells, with similar dose-response curves (2).

In non-transgenic systems, Stephens and Mason showed that cells that prevented diabetes in PVG.RT1u rats were found in both CD25+ and CD25- CD45RC- compartments in the peripheral immune system but not in the thymus, where protective cells were found only in the CD25+ population (5), and; finally, Annacker et al. observed that both CD4+CD25+CD45RBlow and CD4+CD25-CD45RBlow purified populations were able to confer protection from IBD induced by CD45RBhi cells (6). However, we should ask weather CD25- T cells exert its regulatory function as truly CD25-, or there is conversion to CD25+ T cells.

Although CD4+ T cells depleted of CD25+ T cells are capable of regulatory functions, as described above, T cells from animals genetically deficient in CD25 have little or no regulatory activity (7). The main difference between CD4+CD25- cells obtained by depletion and the CD4+CD25- cells from CD25 KO mice is the possibility of CD25 induction in the former but not the latter population. Thus, it looks like CD25 must be expressed at some point for T cells to exert suppression.

Several publications already describe the in vivo conversion of a sizable fraction of CD25- into CD25+ T cells (Bandeira, Almeida, Fontenot). In these reports, CD25+ cells arise from CD25- cells in the absence of other T cells.

Regarding the requirements for conversion, two recent studies have shown that in vitro culture with TGF-beta is sufficient to convert CD25- T cells into CD25+ regulatory T cells.
Recent findings (published this month in the J. Immunol) suggested that CD25 expression on an originally CD25- T cell population depends on the presence of CD25+ cells (8). In this study, Zheng et al. stimulated T cells in vitro with TGF-beta and/or IL-2 to induce the conversion of CD25- cells into CD25+ Treg cells that express FoxP3 (8). When the CD25+ T cells were depleted from the culture, the TGF-beta-mediated enhancement of CD25 (also the IL-2R-alfa-chain) was abolished. The dependence on the presence of CD25+ cells noted in this study contrasts with our previous results of CD25+ depletion in vivo (1, 2) and also with the transfer of purified CD4+CD25- cells into T cell deficient mice, that generated a population of cells expressing CD25 (9-13).

We have recently shown that these CD25+ cells derived in vivo from CD25- T cells express markers of naturally occurring T cells, such as expression of the forkhead box transcription factor Foxp3, GITR, and CTLA-4; furthermore, like naturally occurring CD4+CD25+ Treg cells, the cells that converted in vivo from CD4+CD25- T cells display an anergic state, and suppress the proliferation of other cells in vitro (14). It remains to be determined under which circumstances the conversion of CD4+CD25- cells into regulatory CD4+CD25+ T cells is favored in vivo and also the repertoire requirements for this conversion.

Juan J. Lafaille (lafaille@saturn.med.nyu.edu)
Maria Curotto de Lafaille (curotto@saturn.med.edu)
Daniel Mucida (mucida@saturn.med.edu)


References:
1. Olivares-Villagomez, D., A.K. Wensky, Y. Wang, and J.J. Lafaille. 2000. Repertoire requirements of CD4+ T cells that prevent spontaneous autoimmune encephalomyelitis. J Immunol 164:5499-5507.
2. Curotto de Lafaille, M.A., S. Muriglan, M.J. Sunshine, Y. Lei, N. Kutchukhidze, G.C. Furtado, A.K. Wensky, D. Olivares-Villagomez, and J.J. Lafaille. 2001. Hyper immunoglobulin E response in mice with monoclonal populations of B and T lymphocytes. J Exp Med 194:1349-1359.
3. Gonzalez, M., S.A. Quezada, B.R. Blazar, A. Panoskaltsis-Mortari, A.Y. Rudensky, and R.J. Noelle. 2002. The balance between donor T cell anergy and suppression versus lethal graft-versus-host disease is determined by host conditioning. J Immunol 169:5581-5589.
4. Apostolou, I., A. Sarukhan, L. Klein, and H. von Boehmer. 2002. Origin of regulatory T cells with known specificity for antigen. Nat Immunol 3:756-763.
5. Stephens, L.A., and D. Mason. 2000. CD25 is a marker for CD4+ thymocytes that prevent autoimmune diabetes in rats, but peripheral T cells with this function are found in both CD25+ and CD25- subpopulations. J Immunol 165:3105-3110.
6. Annacker, O., R. Pimenta-Araujo, O. Burlen-Defranoux, and A. Bandeira. 2001. On the ontogeny and physiology of regulatory T cells. Immunol Rev 182:5-17.
7. Furtado, G.C., M.A. Curotto de Lafaille, N. Kutchukhidze, and J.J. Lafaille. 2002. Interleukin 2 signaling is required for CD4(+) regulatory T cell function. J Exp Med 196:851-857.
8. Zheng, S.G., J.H. Wang, J.D. Gray, H. Soucier, and D.A. Horwitz. 2004. Natural and induced CD4+CD25+ cells educate CD4+CD25- cells to develop suppressive activity: the role of IL-2, TGF-beta, and IL-10. J Immunol 172:5213-5221.
9. Annacker, O., R. Pimenta-Araujo, O. Burlen-Defranoux, T.C. Barbosa, A. Cumano, and A. Bandeira. 2001. CD25+ CD4+ T cells regulate the expansion of peripheral CD4 T cells through the production of IL-10. J Immunol 166:3008-3018.
10. Gavin, M.A., S.R. Clarke, E. Negrou, A. Gallegos, and A. Rudensky. 2002. Homeostasis and anergy of CD4(+)CD25(+) suppressor T cells in vivo. Nat Immunol 3:33-41.
11. Furtado, G.C., M.A. Curotto de Lafaille, N. Kutchukhidze, and J.J. Lafaille. 2002. Interleukin 2 signaling is required for CD4(+) regulatory T cell function. J Exp Med 196:851-857.
12. Almeida, A.R., N. Legrand, M. Papiernik, and A.A. Freitas. 2002. Homeostasis of peripheral CD4+ T cells: IL-2R alpha and IL-2 shape a population of regulatory cells that controls CD4+ T cell numbers. J Immunol 169:4850-4860.
13. Fontenot, J.D., M.A. Gavin, and A.Y. Rudensky. 2003. Foxp3 programs the development and function of CD4+CD25+ regulatory T cells. Nat Immunol 4:330-336.
14. Curotto De Lafaille, M.A., Lino, A, Kutchukhidze, N., Lafaille, J. 2004. In vivo conversion of CD4+CD25- cells into Foxp3+ CD25+T cells explains the suppressive function of CD4+CD25- cells. Submitted.