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 the role od cellular competition in lymphocyte selection and immune responses.

4. To study the mechanisms of immunological memory persistence.

To investigate these different issues we have followed several lines of research during 2008:

1- 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 require close vicinity between the interacting CD4 and CD8 T cells.

2. Selection and control of IgM-secreting cells. We studied the fate of mature lymph node (LN) B cells injected into immune-deficient hosts unable to produce B cells. Using this experimental model we found that there are mechanisms of feedback regulation controlling the total number of activated B cells and B cell terminal differentiation. We have found that the IgG produced by the first B cell population controls the production of IgM by the second B cell population. Our findings suggest that the number of activated IgM-secreting B cells may be controlled by quorum-sensing mechanisms: when Ig levels reach a certain threshold, these “signals” are captured by receptors at the B cell surface that inhibit new B cell activation.

3- Endogenous TCR recombination in TCR transgenic Rag-2 deficient mice. The transfer of monoclonal TCR Tg T cells from Rag-2^-/- mice, into allogenic Rag^-/-γc^-/- hosts results in the accumulation in the host mice of donor T cells expressing non-Tg TCRs. Molecular analysis of the expressed TCRs confirmed that these donor T cells expressed a broad diversity of recombined endogenous TCRs. Nucleotide sequence analysis indicates that we are in presence of a “classical” Rag-dependent recombination in spite of the Rag-deficiency of the donors. We found that the T cells expressing non-transgenic TCRs pre-exist in a very limited number both in the thymus and at the periphery of the donor Rag-2^-/- mice.

Keywords: lymphocyte homeostasis, immunological memory, regulatory T cells

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

Sanchez-Guajardo, V., Tanchot, C., O’Malley, J.T., Kaplan, M.H., Garcia, S. & Freitas, A.A. (2007) Agonist-driven development of CD4⁺CD25⁺Foxp3⁺regulatory T cells requires a second signal mediated by Stat6. J. Immunol. 178 : 7550-7556