Bettina Ernst

The Peptide Ligands Mediating Positive Selection in the Thymus Control T Cell Survival and Homeostatic Proliferation in the Periphery

Positive selection to self-MHC/peptide complexes has long been viewed as a device for skewing the T cell repertoire toward recognition of foreign peptides presented by self-MHC molecules. Here, we provide evidence for an alternative possibility, namely, that the self-peptides controlling positive selection in the thymus serve to maintain the longevity of mature T cells in the periphery. Surprisingly, when total T cell numbers are reduced, these self-ligands become overtly stimulatory and cause naive T cells to proliferate and undergo homeostatic expansion.

Interleukin (IL)-15 and IL-7 jointly regulate homeostatic proliferation of memory phenotype CD8+ cells but are not required for memory phenotype CD4+ cells

The overall size and composition of the pool of naive and memory T cells are tightly regulated by homeostatic mechanisms. Recent work has shown that homeostasis of naive T cells is controlled by two factors, self-major histocompatibility complex (MHC)/peptide ligands and a cytokine, interleukin (IL)-7. In particular, contact with these two factors is required for naive CD4+ and CD8+ cells to undergo “homeostatic” proliferation, i.e., proliferation induced as a consequence of severe T cell depletion. In contrast to naive T cells, the factors that drive memory T cells to undergo homeostatic proliferation are poorly understood. To address this issue, purified memory phenotype CD4+ and CD8+ cells from normal mice were adoptively transferred into various gene-knockout mice rendered T cell–deficient by sublethal irradiation. Three findings are reported. First, unlike naive T cells, homeostatic proliferation of memory T cells is largely MHC independent. Second, memory CD8+ cells can utilize either IL-7 or IL-15 to undergo homeostatic proliferation; however, in the absence of both IL-7 and IL-15, homeostatic proliferation fails to occur. Third, unlike memory CD8+ cells, homeostatic proliferation of memory CD4+ cells is independent of IL-7 and IL-15 (also IL-4). Thus, the homeostatic proliferation mechanisms that control memory CD8+ cells and memory CD4+ cells are quite distinct.

β-Catenin Is Dispensable for Hematopoiesis and Lymphopoiesis

β-catenin–mediated Wnt signaling has been suggested to be critically involved in hematopoietic stem cell maintenance and development of T and B cells in the immune system. Unexpectedly, here we report that inducible Cre-loxP–mediated inactivation of the β-catenin gene in bone marrow progenitors does not impair their ability to self-renew and reconstitute all hematopoietic lineages (myeloid, erythroid, and lymphoid), even in competitive mixed chimeras. In addition, both thymocyte survival and antigen-induced proliferation of peripheral T cells is β-catenin independent. In contrast to earlier reports, these data exclude an essential role for β-catenin during hematopoiesis and lymphopoiesis.

Autologous Regulation of Naive T Cell Homeostasis Within the T Cell Compartment

Naive T cells undergo spontaneous slow proliferation on adoptive transfer into syngeneic T cell (T)-deficient hosts. Recent work has shown that such “homeostatic” T cell proliferation is driven by MHC molecules loaded with self-peptides rather than foreign peptides. Because naive T cells in normal T-sufficient hosts remain in interphase despite continuous contact with self-MHC/peptide ligands, T cells apparently inhibit homeostatic proliferation of neighboring T cells. To address this, we have investigated the requirements necessary for “bystander” T cells to inhibit homeostatic proliferation of other T cells. Three key findings are reported. First, homeostatic proliferation of T cells only occurs in specific microenvironments, namely the T cell compartment of the secondary lymphoid tissues. Second, direct entry into T cell compartments is also required for bystander inhibition of homeostatic proliferation. Third, bystander inhibition is mediated largely by naive rather than activated/memory T cells and does not require proliferation or TCR ligation. These findings suggest that homeostasis of naive T cells is unlikely to be regulated through competition for systemic soluble factors or for specific stimulatory self-MHC/peptide ligands. Rather, the data favor mechanisms that involve competition for local non-MHC stimulatory factors or direct cell-to-cell interactions between the T cells themselves within the T cell compartment.

The Role of Somatic Mutation in the Generation of the Protective Humoral Immune Response against Vesicular Stomatitis Virus

During most clinically relevant infections with cytopathic viruses, neutralizing antibodies are generated early, i.e., within the first week of infection. As early as 4 days after immunization of mice with vesicular stomatitis virus (VSV), a cytopathic virus closely related to rabies virus, hybridomas could be isolated that secreted virus-neutralizing IgGs. Such antibodies were devoid of somatic mutations, showed high binding avidities (approximately 10(9) M-1), and used V gene fragments predominantly belonging to the VHQ52 and VK19-28 families. In contrast, most secondary and hyperimmune response IgGs isolated 12 and 150 days after infection used several additional V gene combinations. These, which used the VHQ52/VK19-28 combination of early IgGs, were point mutated but showed only marginally enhanced binding avidities. Since all VHQ52/ VK19-28-positive IgGs bound to one subsite within the major antigenic site of VSV-G irrespective of the presence or absence of somatic point mutations, fine specificity diversification of secondary and hyperimmune responses was achieved by newly appearing V gene combinations.

Dyslipidemia inhibits Toll-like receptor–induced activation of CD8α-negative dendritic cells and protective Th1 type immunity

Environmental factors, including diet, play a central role in influencing the balance of normal immune homeostasis; however, many of the cellular mechanisms maintaining this balance remain to be elucidated. Using mouse models of genetic and high-fat/cholesterol diet–induced dyslipidemia, we examined the influence of dyslipidemia on T cell and dendritic cell (DC) responses in vivo and in vitro. We show that dyslipidemia inhibited Toll-like receptor (TLR)–induced production of proinflammatory cytokines, including interleukin (IL)-12, IL-6, and tumor necrosis factor-α, as well as up-regulation of costimulatory molecules by CD8α− DCs, but not by CD8α+ DCs, in vivo. Decreased DC activation profoundly influenced T helper (Th) cell responses, leading to impaired Th1 and enhanced Th2 responses. As a consequence of this immune modulation, host resistance to Leishmania major was compromised. We found that oxidized low-density lipoprotein (oxLDL) was the key active component responsible for this effect, as it could directly uncouple TLR-mediated signaling on CD8α− myeloid DCs and inhibit NF-κB nuclear translocation. These results show that a dyslipidemic microenvironment can directly interfere with DC responses to pathogen-derived signals and skew the development of T cell–mediated immunity.

Role of self-major histocompatibility complex/peptide ligands in selection and maintenance of a diverse T cell repertoire.

Positive selection has long been thought to be a devise for producing a repertoire of T cells that can efficiently recognize foreign peptides in the context of self-major histocompatibility complex (MHC) molecules. However, in the light of recent evidence that long-term survival of mature T cells requires continuous contact with self-MHC molecules, the possibility for an additional role for positive selection has emerged: to generate a repertoire of T cells that can be maintained in the periphery through contact with self-MHC/peptide ligands. In support of this idea, our recent work suggests that positive selection is highly peptide specific and, more important, that mature T cells require extrathymic contact with the same MHC/peptide ligands that initially induced positive selection in the thymus in order for prolonged survival and to undergo homeostatic proliferation in response to T cell deficiency.

Factors Regulating Naïve T Cell Homeostasis

The mature T cells in the peripheral secondary lymphoid tissues are controlled by homeostatic mechanisms that regulate their survival and the overall cell counts. Such regulation applies to populations of naive and memory T cells. The prevailing view in the past was that the signals essential for regulating T cell homeostasis were thought to be largely derived from contact with foreign antigens. However, recent work from several groups has strongly indicated that homeostasis of naive T cells is crucially dependent on interaction with two self-ligands — self-MHC/peptide complexes and the cytokine IL-7. This article will present our recent work supporting the revised view on the regulation of naive T cell homeostasis.