<p>T lymphocytes are primarily involved in adaptive, cell-mediated, immune reactions. In this thesis T cells were studied regarding central and peripheral differentiation and homeostatic mechanisms for maintanance of the immune repertoire.</p><p>The influence by mature T cells on thymic development was studied in C.B-17 <i>scid/scid</i> (SCID) mice, devoid of mature T and B cells, and whose thymocyte development is arrested at the early pro-T cell stage. When mature syngeneic T cells were injected the developmental block was overcome and there was an accumulation of CD4⁺CD8⁺ thymocytes. This event was accompanied by the maturation of medullary epithelial cells in thymus which seemed to be driven by CD8⁺ T cells. In the periphery there was initially a spontaneous T-cell proliferation and later, the majority of the donor T lymphocytes showed a memory phenotype with high expression of CD44 and with an early onset of proliferation and cytokine production upon stimulation. This stable pool of memory type of cells sustained for more than a year following treatment.</p><p>Treating SCID mice with allogeneic T cells results in graft-versus-host disease (GVHD). Severe GVHD was dependent on the MHC-haplotype of the donor cells and was accompanied by profound alterations of the TCR-Vβ repertoire and with high production of IFN-γ.</p><p>Kappa light chain (κ)-deficient mice have only half the number of B cells as their normal counterparts but normal levels of immunoglobulins. When T cells from κ-deficient mice were stimulated <i>in vitro</i> there was a bias towards production of B-cell stimulatory type 2 cytokines. This is proposed as a mechanism for the homeostatic control of serum immunoglobulin levels in κ-deficient mice.</p>

T lymphocytes are primarily involved in adaptive, cell-mediated, immune reactions. In this thesis T cells were studied regarding central and peripheral differentiation and homeostatic mechanisms for maintanance of the immune repertoire. The influence by mature T cells on thymic development was studied in C.B-17 scid/scid (SCID) mice, devoid of mature T and B cells, and whose thymocyte development is arrested at the early pro-T cell stage. When mature syngeneic T cells were injected the developmental block was overcome and there was an accumulation of CD4+CD8+ thymocytes. This event was accompanied by the maturation of medullary epithelial cells in thymus which seemed to be driven by CD8+ T cells. In the periphery there was initially a spontaneous T-cell proliferation and later, the majority of the donor T lymphocytes showed a memory phenotype with high expression of CD44 and with an early onset of proliferation and cytokine production upon stimulation. This stable pool of memory type of cells sustained for more than a year following treatment. Treating SCID mice with allogeneic T cells results in graft-versus-host disease (GVHD). Severe GVHD was dependent on the MHC-haplotype of the donor cells and was accompanied by profound alterations of the TCR-Vβ repertoire and with high production of IFN-γ. Kappa light chain (κ)-deficient mice have only half the number of B cells as their normal counterparts but normal levels of immunoglobulins. When T cells from κ-deficient mice were stimulated in vitro there was a bias towards production of B-cell stimulatory type 2 cytokines. This is proposed as a mechanism for the homeostatic control of serum immunoglobulin levels in κ-deficient mice.