Self-recognition and the regulation of CD4+ T cell survival

Evidence that the density of self peptide-MHC ligands regulates T-cell receptor signaling

Noncognate or self peptide-MHC (pMHC) ligands productively interact with T-cell receptor (TCR) and are always in a large access over the cognate pMHC on the surface of antigen presenting cells. We assembled soluble cognate and noncognate pMHC class I (pMHC-I) ligands at designated ratios on various scaffolds into oligomers that mimic pMHC clustering and examined how multivalency and density of the pMHCs in model clusters influences the binding to live CD8 T cells and the kinetics of TCR signaling. Our data demonstrate that the density of self pMHC-I proteins promotes their interaction with CD8 co-receptor, which plays a critical role in recognition of a small number of cognate pMHC-I ligands. This suggests that MHC clustering on live target cells could be utilized as a sensitive mechanism to regulate T cell responsiveness.

Mechanisms controlling granule-mediated cytolytic activity of cytotoxic T lymphocytes

Cytotoxic T lymphocytes (CTL) play a critical role in immunity against viruses and cancer. The antigen receptor or T-cell receptor (TCR) on CTL determines the specificity toward target cells. The CD8 co-receptor functions in concert with the TCR to enhance TCR-mediated signaling, accounting for the remarkable sensitivity and swift signaling kinetics of the CTL response. The latter ensures efficient delivery and release of lytic granules, resulting in sensitive and rapid destruction of target cells.

Induction of ssDNA-binding autoantibody secreting B cell immunity during murine malaria infection is a critical part of the protective immune responses

Although it has been hypothesized that autoimmune-like phenomena may play a critical role in the protective immune responses to both human and animal malaria, there are still no evidence-based data to support this view. In this study we demonstrate that the majority of anti-single stranded (ss) DNA autoantibody secreting B cells were confined to B220(+)CD21(+)CD23(-) cells and that these cells expanded significantly in the spleen of C57BL/6 mice infected with Plasmodium yoelii 17X non-lethal (PyNL). To determine the role of ssDNA-binding autoantibody secreting B cell responses in murine malaria, we conjugated generation 6 (poly) amidoamine dendrimer nanoparticles with ssDNA to deplete ssDNA-binding autoreactive B cells in vivo. Our data revealed that 55.5% of mice died after DNA-coated nanoparticle-mediated in vivo depletion of ssDNA-specific autoreactive B cells and subsequent challenge using PyNL. Adoptive transfer of B cells with ssDNA specificity to mice, followed by PyNL infection, caused a later appearance and inhibition of parasitemia. The possible mechanism by which the ssDNA-binding autoantibody secreting B cells is involved in the protection against murine malaria has also been demonstrated.

Self-class I MHC molecules support survival of naive CD8 T cells, but depress their functional sensitivity through regulation of CD8 expression levels

Previous studies have suggested that naive CD8 T cells require self-peptide–major histocompatability complex (MHC) complexes for maintenance. However, interpretation of such studies is complicated because of the involvement of lymphopenic animals, as lymphopenia drastically alters naive T cell homeostasis and function. In this study, we explored naive CD8 T cell survival and function in nonlymphopenic conditions by using bone marrow chimeric donors and hosts in which class I MHC expression is absent or limited to radiosensitive versus radioresistant cells. We found that long-term survival of naive CD8 T cells (but not CD4 T cells) was impaired in the absence of class I MHC. However, distinct from this effect, class I MHC deprivation also enhanced naive CD8 T cell responsiveness to low-affinity (but not high-affinity) peptide–MHC ligands. We found that this improved sensitivity was a consequence of up-regulated CD8 levels, which was mediated through a transcriptional mechanism. Hence, our data suggest that, in a nonlymphopenic setting, self-class I MHC molecules support CD8 T cell survival, but that these interactions also attenuate naive T cell sensitivity by dynamic tuning of CD8 levels.

Modelling naive T-cell homeostasis: consequences of heritable cellular lifespan during ageing

Within an individual, the population of mature naive T cells is maintained throughout life by both input from the thymus and homeostatic proliferation in the periphery. Here, we develop a mathematical model of this process of naive T-cell homeostasis, and use it to explore questions of lifespan, inheritance and receptor repertoire during ageing. By assuming lifespan is largely determined by a heritable trait reset on mitosis, we show that homeostatic proliferation leads naturally to a longer lived population with age. A plausible candidate for the heritable trait influencing lifespan is T-cell receptor affinity for major histocompatibility molecules loaded with self-peptides. Concurrently with increasing lifespan, receptor diversity decreases with age, thus quantitatively linking these two phenomena. These results depend on the thymus involuting with age so that homeostatic proliferation becomes the dominant mode of replacement of the naive T-cell repertoire.

Naive and innate memory phenotype CD4+ T cells have different requirements for active Itk for their development

The Tec family kinase Itk regulates the development of conventional and innate CD8(+) T cells. However, little is known about the role of Itk in the development of CD4(+) T cell lineages, although the role of Itk in the T cell activation and function is well defined. We show in this study that Itk null mice have increased percentage of CD62L(low)CD44(high) memory phenotype CD4(+) T cells compared with wild-type mice. These cells arise directly in the thymus, express high levels of transcripts for the T-bet and IFN-gamma and are able to produce IFN-gamma directly ex vivo in response to stimulation. Itk deficiency greatly decreases the number of CD4(+) T cells with CD62L(high)CD44(low) naive phenotype, but has no effect on the number of memory phenotype CD4(+) T cells, indicating that the development of memory phenotype CD4(+) T cells is Itk-independent. We further show that the development of the naive phenotype CD4(+) T cells is dependent on active Itk signals and can be rescued by expression of Itk specifically in T cells. Our data also show that Itk is required for functional TCR signaling in these cells, but not for the innate function in response to IL-12/IL-18 or Listeria monocytogenes stimulation. These results indicate that CD62L(high)CD44(low) "naive" CD4(+) and CD62L(low)CD44(high) "innate memory phenotype" CD4(+) T cells may be independent populations that differ in their requirement for Itk signals for development. Our data also suggest that CD4(+)CD62L(low)CD44(high) memory phenotype T cells have innate immune function.

Enhanced Levels of Costimulation Lead to Reduced Effector/Memory CD8+ T Cell Functionality

The role of different levels of costimulation in conjunction with signal 1 in the activation of memory CD8+ T cells remains elusive. In this study, we demonstrate, in a mouse model with the influenza nucleoprotein epitope NP68, that mouse early memory (effector/memory) CD8+ T cells that were generated with high levels of costimulation have reduced CTL functionality compared with those that were generated with low levels of costimulation. This reduction is associated with increased phosphorylation of the negative regulatory site 292 on Zap70 and a decrease in granzyme B levels. Furthermore, we show that enhanced costimulation reduces proliferation and cytokine production of effector/memory CD8+ T cells in response to intermediate and weak TCR stimulation, in contrast to previously described positive effects of costimulation on naive CD8+ T cells. This effect is associated with the expression of ICAM-1 on APCs. Together, our results indicate that enhanced costimulation can lead to reduced functionality in effector/memory CD8+ T cells. This compromised effector function of effector/memory CD8+ T cells in response to high levels of costimulation can have important implications for designing immunotherapeutic strategies to enhance immune responses.

Homeostatic expansion permits T cells to re-enter the thymus and deliver antigen in a tolerogenic fashion

We previously have shown that delivery of alloantigen on T cells can be used to induce tolerance through central deletion. Here, we analyzed the requirements for tolerance induced by T cells. Adoptively transferred allogeneic T cells undergo extensive homeostatic proliferation in the periphery of lethally irradiated hosts receiving a syngeneic bone marrow transplant, and acquire a memory-like cell surface phenotype. Analysis of the kinetics of thymic re-entry of transferred T cells revealed that T cells undergo homeostatic proliferation in the periphery prior to re-entry into the thymus. Prevention of homeostatic proliferation results in a failure of transferred T cells to re-enter the thymus. In the absence of homeostatic proliferation, adoptively transferred T cells were unable to induce tolerance. These date suggest that homeostatic proliferation of T cells resulting in an activated cell surface phenotype is required for thymic re-entry and is mechanistically linked to the ability of T cells to induce tolerance.

Self-recognition is crucial for maintaining the peripheral CD4+ T-cell pool in a nonlymphopenic environment

The role of self-recognition in the maintenance of the peripheral CD4+ T-cell pool has been extensively studied, but no clear answer has so far emerged. Indeed, in studies of the role of self-major histocompatibility complex (MHC) molecules in CD4+ T-cell survival, several parameters must be taken into account when interpreting the results: (1) in a lymphopenic environment, observations are biased by concomitant proliferation of T cells arising in MHC-expressing mice; (2) the peripheral T-cell compartment is qualitatively and quantitatively different in nonlymphopenic, normal, and MHC class II-deficient mice; and (3) in C57BL/6 Aβ-/- mice (traditionally considered MHC class II-deficient), the Aα chain and the Eβ chain associate to form a hybrid AαEβ MHC class II molecule. In light of these considerations, we revisited the role of interactions with MHC class II molecules in the survival of peripheral CD4+ T cells. We found that the answer to the question “is self-recognition required for CD4+ T cells to survive?” is not a simple yes or no. Indeed, although long-term survival of CD4+ T cells does not depend on self-recognition in lymphopenic mice, interactions with MHC class II molecules are required for maintaining the peripheral CD4+ T-cell pool in a nonlymphopenic environment. (Blood. 2006;108:270-277)

Lymphocyte homeostasis following therapeutic lymphocyte depletion in multiple sclerosis

Following lymphocyte depletion, homeostatic mechanisms drive the reconstitution of lymphocytes. We prospectively studied this process in 16 patients for 1 year after a single pulse of treatment with Campath-1H, a humanised anti-CD52 monoclonal antibody. We observed two phases of lymphocyte reconstitution. In the first 6 months after treatment the precursor frequency and proliferation index of the patients' autologous mixed lymphocyte reaction increased; the depleted T cell pool was dominated by memory T cells, especially (CD4+)CD25high T cells, a putative regulatory phenotype; and there was a non-significant rise in peripheral mononuclear cell FoxP3 mRNA expression and fall in constitutive cytokine mRNA expression. In the later phase, from 6-to-12 months after Campath-1H, these changes reversed and there was a rise in ROG mRNA expression. However, total CD4+ numbers remained below 50% of pre-treatment levels at 12 months, perhaps reflecting a failure in homeostasis. This was not due to an impaired IL-7 response, as in rheumatoid arthritis, nor to a lack of IL-7 receptors, which are found on fewer human (CD4+)CD25high than naive cells. We speculate that CCL21 and IL-15 responses to lymphopaenia may be suboptimal in multiple sclerosis.

Cutting edge: differential self-peptide/MHC requirement for maintaining CD8 T cell function versus homeostatic proliferation

Memory T cells do not require self-peptide/MHC (spMHC) complexes to survive long term in vivo. However, memory CD4 T cells lose the ability to reject skin grafts when transiently placed in an environment in which these low-level TCR stimulations are absent. Whether or not spMHC alters the ability of CD8 T cells to respond to stimulation in vivo remains unknown. Here, we show that memory CD8 T cells retain the ability to respond to dendritic cell-mediated stimulation after adoptive transfer into either TAP(-/-) (MHC class I-deficient) or wild-type mice. Surprisingly, naive CD8 T cells, which fail to undergo homeostatic proliferation and erode in number in the absence of MHC class I, also retain the ability to respond to dendritic cell-mediated antigenic stimulation for at least 1 wk after transfer into TAP(-/-) mice. These findings suggest a differential requirement for spMHC signals for maintenance of CD8 T cell function and homeostatic proliferation.

Intravital microscopy: visualizing immunity in context

Recent advances in photonics, particularly multi-photon microscopy (MPM) and new molecular and genetic tools are empowering immunologists to answer longstanding unresolved questions in living animals. Using intravital microscopy (IVM) investigators are dissecting the cellular and molecular underpinnings controlling immune cell motility and interactions in tissues. Recent IVM work showed that T cell responses to antigen in lymph nodes are different from those observed in vitro and appear dictated by factors uniquely relevant to intact organs. Other IVM models, particularly in the bone marrow, reveal how different anatomic contexts regulate leukocyte development, immunity, and inflammation. This article will discuss the current state of the field and outline how IVM can generate new discoveries and serve as a "reality check" for areas of research that were formerly the exclusive domain of in vitro experimentation.

Upregulation of the CLIP self peptide on mature dendritic cells antagonizes T helper type 1 polarization

Dendritic cells (DCs) initiate and regulate immunity against foreign and self antigens. Here we identified more than 200 individual major histocompatibility complex class II-associated peptides on human DCs and found that mature DCs selectively upregulated the self peptide CLIP. CLIP cosegregated together with foreign antigenic peptides in tetraspan microdomains on the surface and localized to DC-T cell synapses. The increased representation of CLIP-major histocompatibility complex class II complexes favored polarization of autologous naive T cells toward the nonpolarized and T helper type 2 (T(H)2) phenotype. There was also a considerably higher T(H)2/T(H)1 ratio in H2-DM-deficient mice, which have a CLIP(hi) phenotype, in contrast to wild-type, CLIP(lo) mice. Thus, the self peptide CLIP on DCs qualifies as an endogenous regulator in priming of T helper cells by antagonizing the polarization toward the T(H)1 phenotype.

The role of Tec family kinases in T cell development and function

Three members of the Tec family kinases, Itk, Rlk and Tec, have been implicated in signaling downstream of the T cell receptor (TCR). The activity of these kinases in T cells has been shown to be important for the full activation of phospholipase C-gamma1 (PLC-gamma1). Disruption of Tec family signaling in Itk-/- and Rlk-/-Itk-/- mice has multiple effects on T cell development, cytokine production and T-helper cell differentiation. Furthermore, mice possessing mutations in signaling molecules upstream of PLC-gamma1, such as Src homology 2 (SH2) domain-containing phosphoprotein of 76 kDa (SLP-76), linker for activation of T cells (LAT) and Vav1, or in members of the nuclear factor for activated T cells (NFAT) family of transcription factors, which are downstream of PLC-gamma1, have been found to have similar phenotypes to Tec family-deficient mice, emphasizing the importance of this pathway in regulating T cell activation, differentiation and homeostasis.