The main scientific objectives of the Lymphocyte Population Biology Unit are:

1. To study the mechanisms of homeostasis, which control the number of B and T lymphocytes.

2. To study the dynamics of the lymphocyte populations: rates of cell production and cell death, mechanisms of lymphocyte survival.

3. To study to what extent can cellular competition contribute to lymphocyte selection and to the control of immune responses.

4. To study which mechanisms induce persistence of immunological memory.

To investigate these different issues we have followed several lines of research. We summarize our most important observations during 2006:

1- Endogenous TCR recombination in TCR transgenic Rag-2 deficient mice. The transfer of monoclonal TCR Tg T cells from H2^k 5CC7 Rag-2^-/- mice, which are specific for the pigeon cytochrome C, into allogenic H2^b Rag^-/-γc^-/- hosts resulted in the accumulation in the host mice of donor T cells expressing non-Tg TCRs. Molecular analysis of the expressed TCRs by Immunoscope confirmed that these donor T cells expressed a broad diversity of recombined endogenous TCRs. Nucleotide sequence analysis of the expressed non-Tg TCR indicates that we are in presence of a mechanism of “classical” Rag-dependent recombination in spite of the Rag-2 deficiency of the 5CC7 donors. We found that T cells expressing a non-transgenic TCR pre-exist in a very limited number both in the thymus and at the periphery of the naive 5CC7 Rag-2^-/- mice. These results have important implications for the studies using TCR Rag^-/- transgenic mice.

2- TCR specificity and clonal competition. We asked to which extend TCR specificity determines clonal competition for proliferation and/or survival during lymphopenia driven proliferation (LDP). We found that resident monoclonal T cells in TCR Tg Rag^-/- mice, or monoclonal LDP derived TCR Tg T cells in Rag^-/- hosts, inhibit the survival and/or the proliferation of T cells presenting the same TCR, but not of TCR Tg T cells bearing a different specificity. Using different transfer approaches we extended this notion to polyclonal T cells. Our findings show that T TCR-specificity determines peripheral T cell fate and indicate that specific sp-MHC complexes are limiting resources shared between developing, surviving and proliferating T cells.

3- Bystander CD4⁺ T cell help to CD8⁺ T cells during lymphopenia driven proliferation (LDP). Since a fully functioning immune system requires a variety of lymphocyte sub-sets, lymphpocyte homeostasis should control both absolute numbers and relative sizes of each sub-population; otherwise, deregulation and disease may occur. We studied CD8:CD4 T cell interactions during LDP. We found that the co-transfer of CD8⁺ T cells sub-sets with naïve CD4⁺ cells results in the 10-fold increase of the number of CD8⁺ T cells recovered irrespectively of the CD8 T cell sub-set transferred. This “bystander helper” effect results in the preferential accumulation of cells with a TEM phenotype. The mechanisms that mediate the CD4 bystander helper effect are currently under investigation.

Keywords: lymphocyte homeostasis / immunological memory / regulatory T cells

Gaudin, E., Hao, Y., Rosado, M.M., Chaby, R., Girard, R. & Freitas, A.A. (2004) Positive selection of B cells expressing low densities of self-reactive BCRs. J. Exp. Med. 199 : 843-853

Sanchez-Guajardo, V., Borghans, J.A.M., Marquez, M.-E., Garcia, S. & Freitas, A.A. (2005) Different competitive capacities of Stat4- and Stat6-deficient CD4⁺T cells during lymphopenia-driven proliferation. J. Immunol. 174 : 1178-1187

De La Coste, A., Six, E., Fazilleau, N., Mascarell L., Legrand, N., Mailhe, M.-P., Cumano, A., Laabi, Y. & Freitas, A.A. (2005) In vivoand in absence of a thymus the enforced expression of the Notch ligands Delta-1 or Delta-4 promotes T cell development with specific unique effects. J. Immunol.174 : 2730-2737

Almeida ARM, Zaragoza B. & Freitas AA. (2006) Indexation as a novel mechanism of lymphocyte homeostasis: the number of CD4⁺CD25⁺regulatory T cells is indexed to the number of IL-2 producing cells. J. Immunol. 177 : 192-200

Hao Y., Legrand L. & Freitas AA. (2006) The clone size of peripheral CD8 T cells is regulated by TCR promiscuity. J. Exp. Med. 203 : 1643-1649