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

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

To study the role of cellular competition in lymphocyte selection and immune responses.

To study the mechanisms of immunological memory persistence.

In 2009 we followed several lines of research:

1- Bystander CD4⁺ T cell help to CD8⁺ T cells during lymphopenia driven proliferation (LDP). We studied the fate of selected populations of CD8⁺ and CD4⁺ T cells in T cell deficient CD3ε^-/- mice. We found that the reconstitution of the CD8⁺ T cell pool is independent of the nature of the CD8⁺ T cells transferred, suggesting that the resulting pools are environmentally controlled. However, co-transfer of CD8 T cells with CD4⁺ T cells modifies CD8⁺ T cell recovery - results in the dramatic increase of the CD8⁺ T cell numbers recovered. This "helper" effect generates preferentially an increased number of CD8 T cells expressing a TEM phenotype and cytotoxic effector molecules and is does not alter the number of cells with a TCM phenotype. We showed that during LDP bystander CD4 T cell help did not involve CD40 expression by the expanding CD8 T cells, but required CD40 expression by host non-lymphoid cells. Using cells from mice invalidated for the CCR5 molecule we showed that the helper effects also require close vicinity between the interacting CD4 and CD8 T cells. Moreover the bystander helper effects were dependent on IL-2 produced by the expanding CD4⁺ T cells and required expression of IL-2Rb chain but not of the IL-2Ra chain by the responding CD8⁺ T cells. Thus, plasticity on the TEM-phenotype CD8⁺ T cell niche contrasts with stringent homeostatic mechanisms in TCM-phenotype CD8⁺ T cell numbers and points to different homeostatic control mechanisms for TCM and TEM-phenotype 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 Rag° hosts. We found that a fraction of the transferred population of LN B cells expanded and persisted for prolonged periods of time. A significant fraction of the surviving B cells express an activated MZ B cell phenotype and were actively engaged in IgM-secretion. Serum IgM concentrations identical to those of control mice were readily reached in the presence of a reduced number of B cells. We investigated different aspects of the biology of the natural IgM-secreting cells. We found that mechanisms of feedback regulation control the number of activated B cells. We have found that the IgG produced by the first B cell population controls the production of IgM by the second B cell populations. Mouse IgG passively administered into Rag-deficient hosts strongly inhibits the activation and IgM production by adoptively transferred B cells. More recently, we found that B cells from FcγRIIB^-/- donors are not suppressed. These findings suggest that the number of activated IgM-secreting cells may be controlled by quorum-sensing mechanisms and that when the serum Ig levels reach a determined threshold, these "signals" are captured by receptors at the B cell surface that inhibit B cell activation.

3- The homeostasis of the IL-2 producing T cells.

We have shown that the interactions between the CD4⁺CD25⁺ regulatory T cells and naïve CD25^-CD4⁺ T cells are of major relevance for the establishment of peripheral CD4 T cell homeostasis. We demonstrated that the IL-2Ra is an absolute requirement for the generation of the regulatory cells. The expression of the high-affinity IL-2Ra endows these cells with the capacity to explore the IL-2 resource, which ensures their peripheral survival, while keeping their number tied to the number of CD4⁺ T cells that produce IL-2. The indexing of CD4⁺CD25⁺Foxp3⁺ Treg cells to the number of activated IL-2-producing CD4⁺ T cells may constitute a feedback mechanism that controls T cell expansion during immune responses, thus preventing autoimmune or lymphoproliferative diseases. These results indicated that the number of IL-2-producing cells is relevant for regulatory T cells homeostasis as they may control their maintenance in the peripheral pools. These findings indicate that a quorum-sensing feedback loop, where the IL-2 produced by T cell sub-population is detected by a sub-population of CD4 Treg cells expressing the high-affinity IL-2Ra-chain that controls the number of total CD4 T cells. That is to say: overall CD4 T cell populations adapt their behavior according to the detection of the quantities of IL-2 produced. We are currently investigating The properties and homeostasis of IL-2 producing (IL-2p) T cells.

Keywords: lymphocyte homeostasis, immunological memory, regulatory T cells

Lampropoulou, V., Hoehlig, K., Roch, T., Neves, P., Calderon Gomez, E., Sweenie, C.H., Hao, Y., Freitas, A.A., Steinhoff, U., Anderton, S.M. & Fillatreau, S. (2008) TLR- activated B cells suppress T cell-mediated autoimmunity. J. Immunol. 180 : 4763-4773

Freitas, A.A. & Rocha B. (2009) Homeostasis of naive T cells : the Foxo that fixes. Nature Immunol. 10 : 133-144

Leitao, C., Freitas, A.A. & Garcia, S. (2009) The role of TCR specificity and clonal competition during reconstruction of the peripheral T cell pool. J Immunol. 182 : 5232-5239

Montaudouin, C., Boucontet, L., Mailhe-Lembezat, M.-P., Mariotti-Ferrandiz, M.-E., Louise, A., Six, A., Freitas, A.A. & Garcia, S. (2010) Endogenous TCR recombination in TCR Tg single Rag-deficient mice uncovered by robust in vivo T cell activation and selection. PLoS ONE 5 : e10238

Lalanne, A.I., Moraga, I., Hao, Y., Pereira, J.P., Alves, N.L., Huntington, N.D., Freitas, A.A., Cumano, A. & Vieira, P. (2010) CpG inhibits Pro-B cell expansion through a cathepsin B-dependent mechanism. J. Immunol. 184 : 5678-5685