Govindarajan Thangavelu

Programmed death-1 is required for systemic self-tolerance in newly generated T cells during the establishment of immune homeostasis

Lymphopenia driven T cell activation is associated with autoimmunity. That lymphopenia does not always lead to autoimmunity suggests that control mechanisms may exist. We assessed the importance of the co-inhibitory receptor programmed death-1 (PD-1) in the control of lymphopenia-driven autoimmunity in newly generated T cells vs. established peripheral T cells and in thymic selection. PD-1 was not required for negative selection in the thymus or for maintenance of self tolerance following transfer of established PD-1⁻/⁻ peripheral T cells to a lymphopenic host. In contrast, PD-1 was essential for systemic self tolerance in newly generated T cells under lymphopenic conditions, as PD-1⁻/⁻ recent thymic emigrants (RTE), generated after transfer of PD-1⁻/⁻ hematopoietic stem cell (HSC) precursors or thymocytes into lymphopenic adult Rag⁻/⁻ recipients, induced a rapidly lethal multi-organ inflammatory disease. Disease could be blocked by using lymph node deficient recipients, indicating that lymphopenia driven PD-1⁻/⁻ T cell activation required access to sufficient lymph node stroma. These data suggested that PD-1⁻/⁻ mice themselves might be substantially protected from autoimmunity because their T cell repertoire is first generated early in life, a period naturally deficient in lymph node stroma. Consistent with this idea, neonatal Rag⁻/⁻ recipients of PD-1⁻/⁻ HSC were resistant to disease. Thus, a critical role of PD-1 resides in the control of RTE in lymphopenia. The data suggest that PD-1 and a paucity of lymphoid stroma cooperate to control autoimmunity in newly generated T cells. Clinical therapies for autoimmune disease employing lymphoablation and hematopoietic stem cell transplantation will need to take into account functional polymorphisms in the PD-1 pathway, if the treatment is to ameliorate rather than exacerbate autoimmunity.

BTLA targeting modulates lymphocyte phenotype, function, and numbers and attenuates disease in nonobese diabetic mice

The novel coinhibitory receptor BTLA may have a regulatory role in maintaining peripheral tolerance; however, its role in autoimmune diabetes is unknown. In this study, we show that anti‐BTLA mAb 6F7 selectively depleted pathogenic B and CD4+ TH cells; enhanced the proportion of cells with the forkhead box p3+ PD‐1+CD4+ regulatory T phenotype; and increased the production of potentially protective (IL‐10) and detrimental (IL‐2, IFN‐γ) cytokines in NOD mice. As interactions between BTLA and PD‐1 coinhibitory pathways have been described in the cardiac allograft model, we also investigated if these pathways may have significant interaction in autoimmune diabetes. Anti‐BTLA inhibited anti‐PD‐1‐potentiated total IL‐12 (p40+p70) production, suggesting the possibility that anti‐BTLA may have a greater effect in the setting of anti‐PD‐1‐triggered diabetes. To test this, NOD mice at 4 and 10 weeks of age were treated with anti‐BTLA mAb, anti‐PD‐1 mAb, both mAb, or isotype control and were monitored for diabetes development. Although anti‐BTLA mAb delayed diabetes onset significantly in 10‐ but not 4‐week‐old NOD mice, anti‐BTLA mAb attenuated anti‐PD‐1‐induced diabetes in both age groups. Hence, strategies targeting BTLA+ lymphocytes or therapies enhancing the BTLA‐negative cosignal may prove valuable in treating autoimmune diabetes.

PD‐1 is not required for natural or peripherally induced regulatory T cells: Severe autoimmunity despite normal production of regulatory T cells

The expression of the coinhibitor PD‐1 on T cells is important for the establishment of immune homeostasis. We previously found that PD‐1 is particularly critical for the control of self‐tolerance during lymphopenia‐induced proliferation of recent thymic emigrants (RTEs). Previous studies suggested that PD‐1 modulates the generation of Treg cells, particularly peripherally induced Treg (pTreg) cells, and controls Th17 cells. However, these conclusions were derived indirectly from studies on the ligand PD‐L1, and not PD‐1 itself. Herein we directly tested whether T‐cell PD‐1 expression was needed for Treg cell generation and examined if a paucity of Treg cells or enhanced Th17 cells could explain the severe lymphopenia‐potentiated autoimmunity caused by PD‐1 KO RTEs. Employing the murine FoxP3EGFP reporter system to simultaneously monitor conversion of WT and PD‐1 KO T cells to pTreg cells in the same animal, we found that PD‐1 deficiency did not inhibit pTreg cell generation or lead to Th17‐cell‐mediated autoimmunity. Surprisingly, pTreg cell numbers were increased in PD‐1 KO versus WT cell populations. Furthermore, we noted an increased conversion to pTreg cells by RTEs. Our data suggest that the primary role for PD‐1 is to restrain T‐cell activation/proliferation to self‐Ags rather than promote generation of Treg cells.

Control of In Vivo Collateral Damage Generated by T Cell Immunity

An ongoing dilemma faced during an immune response is generating an effective, often proinflammatory response to eliminate pathogens and/or infected cells while also minimizing collateral damage to adjacent noninfected tissues. The factors limiting bystander cell injury during an Ag-specific immune response in vivo are largely unknown. In this study, using an in vivo model of islet transplants in TCR transgenic mice, we show that both CD4 and CD8 T cells do have the capacity to inflict adjacent tissue damage and that this injury is greatly enhanced in sensitized hosts. CD4 T cell–mediated killing of specific and bystander cells occurred via different mechanisms. Unlike specific target cell killing, CD4-mediated bystander injury required tissue Fas expression and was inhibited with anti–IFN-γ Ab treatment in vivo. Moreover, bystander cell injury was not entirely nonspecific but rather required, in naive recipients, that the MHC allele expressed by the bystanders was self. Importantly, the coinhibitor programmed death-1 plays an important role in restraining bystander cell injury mediated either by defined TCR transgenic T cells or by polyclonal T cell populations. Thus, the differential requirements for specific versus bystander cell injury suggest that there are opportunities for inhibiting immune pathology without compromising Ag-specific immunity in vivo.

Prior to Peripheral Tolerance, Newly Generated CD4 T Cells Maintain Dangerous Autoimmune Potential: Fas- and Perforin-Independent Autoimmunity Controlled by Programmed Death-1

Lymphopenia can result from various factors, including viral infections, clinical interventions, or as a normal property of the fetal/neonatal period. T cells in a lymphopenic environment undergo lymphopenia-induced proliferation (LIP) to fill the available “niche” as defined by peptide–MHC and homeostatic cytokine resources. We recently reported systemic autoimmunity following reconstitution of the lymphoid compartment of Rag1−/− mice with PD-1−/− hematopoietic stem cells or by transfer of thymocytes, but not splenocytes, suggesting that programmed death-1 (PD-1) plays a crucial role in controlling recent thymic emigrants (RTE) and preventing autoimmunity upon their LIP. However, it is unclear whether RTE residing within the periphery of a lymphoreplete host maintain enhanced autoimmune generating potential or if this property only manifests if RTE experience a lymphopenic periphery immediately after export from the thymus. Furthermore, it is unclear which of a variety of T cell effector mechanisms generate pathology when control of RTE by PD-1 is lacking. Herein, we determined that PD-1 is upregulated on CD4 T cells undergoing the natural LIP characteristic of the neonatal period. Newly generated T cells lacking PD-1 maintained an enhanced autoimmune potential even after residence in a lymphoreplete periphery, emphasizing the importance of PD-1 in the establishment of peripheral tolerance. Neither Fas nor perforin-dependent killing mechanisms were required for autoimmunity, while host MHC-II expression was critical, suggesting that LIP-driven autoimmunity in the absence of PD-1 may primarily result from a CD4 T cell-mediated systemic cytokinemia, a feature potentially shared by other autoimmune or inflammatory syndromes associated with immune reconstitution and LIP.

Pregnancy per AI and pregnancy loss in lactating dairy cows of a single herd following timed artificial insemination or insemination at detected estrus

Thangavelu, G., Gobikrushanth, M., Colazo, M. G. and Ambrose, D. J. 2015. Pregnancy per artificial insemination and pregnancy loss in lactating dairy cows of a single herd following timed artificial insemination or insemination at detected estrus. Can. J. Anim. Sci. 95: 383–388. The objective of this retrospective study was to determine the factors affecting pregnancy per artificial insemination (P/AI) and pregnancy loss in lactating dairy cattle. Breeding records (n=1466) for 5 consecutive years were evaluated from one dairy herd. The effects of type of breeding [timed artificial insemination (TAI; n=1246) vs. insemination at detected estrus (IDE; n=220)], parity (primiparous vs. multiparous), body condition score (BCS; low ≤2.5 vs. high >2.5), year, season (summer vs. other seasons) and fertility group (high fertile <3 vs. low fertile ≥3 inseminations), on P/AI and pregnancy loss (i.e., late embryonic/early fetal loss, abortion and stillbirth) were determined using the GLIMMIX procedures of SAS software...

An essential role for programmed death-1 in the control of autoimmunity: implications for the future of hematopoietic stem cell transplantation

Future Oncol. (2011) 7(8), 929–932 Co-inhibitory receptors are expressed by lymphocytes and other immune system cells. Their central function is to control the activation of the lymphocyte responses by providing negative signals in conjunction with signals from lymphocyte antigen receptors (‘cosignaling’). Recently, co-inhibitory molecules have been extensively studied due to the broad spectrum of their potential application in various novel immunotherapeutic approaches. The cosignaling aspect of these receptors is particularly appealing for clinical development, as non depleting antibodies targeting these co-inhibitory receptors will only affect lymphocytes that are encountering (or have recently encountered) their cognate antigen; ligation of co-inhibitors alone, without antigen receptor signals, has no effect. Even though it is now 40 years since the initial concept of co-inhibitory signaling was pioneered [1] (and reviewed in [2]), only recently have these signaling pathways been specifically targeted for therapy. Agonistic monoclonal antibodies to co-inhibitors are a promising new approach in the prevention of transplant rejection and graftversus-host disease, and may also find application in treating autoimmune disease. Monoclonals that instead block these co-inhibitors are currently undergoing clinical trials in cancer treatment, and one such co-inhibitory blocker, specific to cytotoxic T-lymphocyte antigen 4, has recently achieved US FDA approval. Programmed death-1 (PD-1; CD279), is another important co-inhibitor. It is involved in the regulation of immune responses and self-tolerance [3]. PD-1 has two known ligands: PD-L1 (B7H1; CD274) and PD-L2 (B7DC; CD273). A recent study suggested that PD-1 may have a specific role in controlling T cells during lymphopenia, although a requirement for PD-1 in the control of lymphopenia-driven autoimmunity was not examined [4]. Autoimmunity can only rarely be attributed to variants of a single gene alone. Instead, discovery of the causes of most autoimmunity will be found in synergisms between alleles of many different genes and their interactions with the environment.

Divide and conquer: Blocking graft versus host but not graft versus leukemia T cells with agonist BTLA co-inhibitory signals.

One of the main objectives in allogeneic hematopoietic stem cell transplantation (aHSCT) research is the prevention of graft versus host disease (GVHD) while maintaining the graft versus leukemia/lymphoma (GVL) effect. Whether these two responses generated by donor T cells can be sufficiently separated and controlled remains controversial. While various approaches have been tested to achieve this goal, success has been relatively limited. Lymphocyte responses are negatively regulated by a series of receptors that function along with antigen receptors to deliver co-inhibitory signals. B and T lymphocyte associated (BTLA) is a novel co-inhibitory molecule expressed by activated T cells, B cells and other immune cells. A study by Albring et al. has now shown in a murine model that a single injection of agonistic anti-BTLA monoclonal antibody can inhibit GVHD long-term while maintaining GVL responses and immunity to infection. These studies suggest that future development of biologics to harness the function of co-inhibitory signals will be an important approach in the prevention of autoimmunity and GVHD and in protocols to achieve transplantation tolerance.