Confounding factors complicate conclusions in aly model

Innate immunity alone is not sufficient for chronic rejection but predisposes healed allografts to T cell-mediated pathology

BACKGROUND

Acute allograft rejection is dependent on adaptive immunity, but it is unclear whether the same is true for chronic rejection. Here we asked whether innate immunity alone is sufficient for causing chronic rejection of mouse cardiac allografts.

METHODS

We transplanted primarily vascularized cardiac grafts to recombinase activating gene-knockout (RAG(-/-)) mice that lack T and B cells but have an intact innate immune system. Recipients were left unmanipulated, received adjuvants that stimulate innate immunity, or were reconstituted with B-1 lymphocytes to generate natural IgM antibodies. In a second model, we transplanted cardiac allografts to mice that lack secondary lymphoid tissues (splenectomized aly/aly recipients) and studied the effect of NK cell inactivation on T cell-mediated chronic rejection.

RESULTS

Acute cardiac allograft rejection was not observed in any of the recipients. Histological analysis of allografts harvested 50 to 90 days after transplantation to RAG(-/-) mice failed to identify chronic vascular or parenchymal changes beyond those observed in control syngeneic grafts. Chronic rejection of cardiac allografts parked in splenectomized aly/aly mice was observed only after the transfer of exogenously activated T cells. NK inactivation throughout the experiment, or during the parking period alone, reduced the severity of T cell-dependent chronic rejection.

CONCLUSIONS

The innate immune system alone is not sufficient for causing chronic rejection. NK cells predispose healed allografts to T cell-dependent chronic rejection and may contribute to chronic allograft pathology.

B-cells need a proper house, whereas T-cells are happy in a cave: the dependence of lymphocytes on secondary lymphoid tissues during evolution

A fundamental tenet of immunology is that adaptive immune responses are initiated in secondary lymphoid tissues. This dogma has been challenged by several recent reports. We discuss how successful T cell-mediated immunity can be initiated outside of such dedicated structures, whereas they are required for adaptive humoral immunity. This resembles an ancient immune pathway in the oldest cold-blooded vertebrates, which lack lymph nodes and sophisticated B-cell responses including optimal affinity maturation. The T-cell, however, has retained the capacity to recognize antigen in a lymph node-free environment. Besides bone marrow and lung, the liver is one organ that can potentially serve as a surrogate lymphoid organ and could represent a remnant from the time before lymph nodes developed.

Requirement of secondary lymphoid tissues for the induction of primary and secondary T cell responses against Listeria monocytogenes

Activation of naive T cells is tightly controlled and depends on cognate interactions with professional antigen-presenting cells. We analyzed dependency on secondary lymphoid tissues for the activation of naive and memory CD4(+) and CD8(+) T cells following primary and secondary Listeria monocytogenes infection, respectively. In splenectomized lymphotoxin-beta receptor-deficient mice, lacking all secondary lymphoid tissues, oral infection with L. monocytogenes failed to induce bacteria-specific CD4(+) and CD8(+) T cell responses. Treatment of splenectomized wild-type mice with FTY720, a drug that prevents egress of T cells from lymph nodes, also reduced T cell responses after oral L. monocytogenes infection and blocked T cell responses after intravenous infection. FTY720-treated wild-type and lymphotoxin-beta receptor-deficient mice show only slightly impaired recall responses. However, T cell responses were profoundly inhibited when mice were splenectomized subsequently to recovery from primary infection. T cell transfer experiments demonstrated that the impaired secondary T cell response was not simply due to removal of a large fraction of memory T cells by splenectomy. Overall, these results indicate that not only primary T cell responses, but also secondary T cell responses, highly depend on the lymphoid environment for effective activation.

Prevention of acute graft-versus-host disease by blocking T-cell entry to secondary lymphoid organs

In acute graft-versus-host disease (aGVHD), donor T cells attack the recipient's gastrointestinal tract, liver, and skin. We hypothesized that blocking access to distinct lymphoid priming sites may alter the specific organ tropism and prevent aGVHD development. In support of this initial hypothesis, we found that different secondary lymphoid organs (SLOs) imprint distinct homing receptor phenotypes on evolving alloreactive effector T cells in vivo. Yet preventing T-cell entry to specific SLOs through blocking monoclonal antibodies, or SLO ablation, did not alter aGVHD pathophysiology. Moreover, transfer of alloreactive effector T cells into conditioned secondary recipients targeted the intestines and liver, irrespective of their initial priming site. Thus, we demonstrate redundancy of SLOs at different anatomical sites in aGVHD initiation. Only prevention of T-cell entry to all SLOs could completely abrogate the onset of aGVHD.

Deletion of DOCK2, a regulator of the actin cytoskeleton in lymphocytes, suppresses cardiac allograft rejection

Allograft rejection is induced by graft tissue infiltration of alloreactive T cells that are activated mainly in secondary lymphoid organs of the host. DOCK2 plays a critical role in lymphocyte homing and immunological synapse formation by regulating the actin cytoskeleton, yet its role in the in vivo immune response remains unknown. We show here that DOCK2 deficiency enables long-term survival of cardiac allografts across a complete mismatch of the major histocompatibility complex molecules. In DOCK2-deficient mice, alloreactivity and allocytotoxicity were suppressed significantly even after in vivo priming with alloantigens, which resulted in reduced intragraft expression of effector molecules, such as interferon-γ, granzyme B, and perforin. This is mediated, at least in part, by preventing potentially alloreactive T cells from recruiting into secondary lymphoid organs. In addition, we found that DOCK2 is critical for CD28-mediated Rac activation and is required for the full activation of alloreactive T cells. Although DOCK2-deficient, alloreactive T cells were activated in vitro in the presence of exogenous interleukin-2, these T cells, when transferred adoptively, failed to infiltrate into the allografts that were transplanted into RAG1-deficient mice. Thus, DOCK2 deficiency attenuates allograft rejection by simultaneously suppressing multiple and key processes. We propose that DOCK2 could be a novel molecular target for controlling transplant rejection.

Asialo GM1 Positive CD8+ T Cells Induce Skin Allograft Rejection in the Absence of the Secondary Lymphoid Organs1

BACKGROUND

Secondary lymphoid organs are considered to be the only organs in which APCs and naïve T cells interact to initiate adaptive immune responses. Aly/aly mice are autosomal recessive mutants of C57BL/6 mice, and lack lymph nodes and Peyer's patches. In this study, we investigated immune responses to skin allografts in splenectomized aly/aly mice, which lack secondary lymphoid organs completely, and examined the effect of anti-asialo GM1 (AsGM1) antibodies on these responses.

METHODS

Skin allografts were transplanted to 1) heterozygous aly/+ mice, which had normal secondary lymphoid organs, 2) splenectomized aly/+ mice, 3) aly/aly mice, and 4) splenectomized aly/aly mice, with and without anti-AsGM1 antibody treatment. Graft survival time and alloreactive antibody production were investigated.

RESULTS

Heterozygous aly/+ mice and splenectomized aly/+ mice rejected skin allografts acutely. Aly/aly mice also rejected skin allografts, but at a later time than aly/+ mice. Sixty percent of splenectomized aly/aly mice rejected skin allografts within 120 days. Serial administration of anti-AsGM1 antibodies prevented skin allograft rejection in splenectomized aly/aly mice during the same 120-day period of observation. After ceasation of anti-AsGM1 antibody treatment, skin allografts were rejected; we observed a simultaneous increase in AsGM1 expression on CD8+ T cells. Alloreactive antibodies were detected in both splenectomized aly/aly mice that rejected skin allografts and in splenectomized aly/aly mice that accepted skin allografts after treatment with anti-AsGM1 antibodies.

CONCLUSIONS

Cytotoxic and humoral immune responses to skin allografts could be initiated despite the absence of secondary lymphoid organs. AsGM1+ cells were important effector cells in secondary lymphoid organ-independent skin allograft rejection.

The site of primary T cell activation is a determinant of the balance between intrahepatic tolerance and immunity

Hepatic immunobiology is paradoxical: although the liver possesses unusual tolerogenic properties, it is also the site of effective immune responses against multiple pathogens and subject to immune-mediated pathology. The mechanisms underlying this dichotomy remain unclear. Following previous work demonstrating that the liver may act as a site of primary T cell activation, we demonstrate here that the balance between immunity and tolerance in this organ is established by competition for primary activation of CD8+ T cells between the liver and secondary lymphoid tissues, with the immune outcome determined by the initial site of activation. Using a transgenic mouse model in which antigen is expressed within both liver and lymph nodes, we show that while naive CD8+ T cells activated within the lymph nodes were capable of mediating hepatitis, cells undergoing primary activation within the liver exhibited defective cytotoxic function and shortened half-life and did not mediate hepatocellular injury. The implications of these novel findings may pertain not only to the normal maintenance of peripheral tolerance, but also to hepatic allograft tolerance and the immunopathogenesis of chronic viral hepatitis.

Parenchymal cells in immune and tolerance induction

There is increasing evidence that dendritic cells are capable of inducing T cell tolerance to tissue-specific antigens by presenting these antigens to CD4 and CD8 T cells in the respective draining lymph nodes. In contrast, parenchymal cells are often seen as immunologically inert. This lecture summarizes studies showing that tissue cells like sinusoidal endothelial cells and hepatocytes in the liver as well as keratinocytes in the skin can induce peripheral T cell tolerance. This tolerance induction is often preceded by T cell activation. Thus, several tolerance mechanisms are operating in parallel. We still have no learn which mechanism is most suitable for therapeutic intervention in autoimmune diseases and organ transplantation.

PROLONGATION OF ALLOGRAFT SURVIVAL IN CCR7-DEFICIENT MICE

BACKGROUND

Lymphocyte homing to secondary lymphoid organs is thought to be required for initiation of the alloreactive immune response. Because CCR7 is the essential chemokine receptor responsible for lymphocyte and dendritic cell homing to secondary lymphoid organs, allograft survival was analyzed in CCR7-deficient (CCR7) mice.

METHODS

Heterotopic heart and skin allotransplantation was performed in CCR7 and wild-type (WT) recipients. Graft survival was monitored daily. Grafts and draining lymph nodes were analyzed by immunohistology and flow cytometry at different time points. Groups of mice were splenectomized at the day of allotransplantation.

RESULTS

A significant though modest prolongation of allograft survival in CCR7 recipients was observed for heart grafts (WT, 7.3 +/- 0.5 days; CCR7, 10.7 +/- 2.8 days) and skin grafts (WT, 8.9 +/- 0.9 days; CCR7, 12.3 +/- 0.9 days). This was accompanied by a delay in the cellular infiltration of allografts. T-cell accumulation and expansion in the draining lymph nodes in CCR7 recipients was severely impaired. Splenectomy had only a moderate prolongation effect on allograft survival in CCR7 mice.

CONCLUSIONS

These results suggest that CCR7-dependent processes support allograft rejection yet are dispensable for the rejection response.

Bone marrow as a priming site for T-cell responses to blood-borne antigen

Although bone marrow is known as a primary lymphoid organ, its potential to serve as a secondary immune organ has hardly been explored. Here we demonstrate that naive, antigen-specific T cells home to bone marrow, where they can be primed. Antigen presentation to T cells in bone marrow is mediated via resident CD11c+ dendritic cells. They are highly efficient in taking up exogenous blood-borne antigen and processing it via major histocompatibility complex class I and class II pathways. T-cell activation correlates with dendritic cell-T cell clustering in bone marrow stroma. Primary CD4+ and CD8+ T-cell responses generated in bone marrow occur in the absence of secondary lymphoid organs. The responses are not tolerogenic and result in generation of cytotoxic T cells, protective anti-tumor immunity and immunological memory. These findings highlight the uniqueness of bone marrow as an organ important for hemato- and lymphopoiesis and for systemic T cell-mediated immunity.

Secondary lymphoid organs are important but not absolutely required for allograft responses

The role of secondary lymphoid organs in adaptive immune responses following transplantation is controversial. To examine the requirement for peripheral lymphoid organs in mounting immune responses to transplantation antigens, lymphotoxin alpha-deficient (LTalpha-/-) and LTbeta-receptor-deficient (LTbetaR-/-) mice that lack lymph nodes and Peyer's patches were used as recipients of fully allogeneic heart and skin grafts. Splenectomized LTalpha-/- and LTbetaR-/- mice effectively rejected skin and cardiac allografts, although with delayed kinetics when compared with wild-type controls. In addition, initial skin allograft challenge in splenectomized LTbetaR-/- mice resulted in accelerated rejection of subsequent donor cardiac allografts when compared with heart rejection in nonsensitized controls. Thus, although peripheral lymphoid organs play an important role in allowing allograft responses to occur, they do not appear to be absolutely required for either acute allograft rejection, or T-cell priming. These results suggest that immunologic events capable of leading to allograft rejection can successfully occur at sites other than classical secondary lymphoid organs.

Non-hematopoietic allograft cells directly activate CD8+ T cells and trigger acute rejection: an alternative mechanism of allorecognition

Despite evidence that human non-hematopoietic cells, such as vascular endothelium, can activate allogeneic T lymphocytes in vitro, the prevailing view has been that hematopoietic antigen-presenting cells are required to trigger alloimmune responses in vivo. Here we report that mouse non-hematopoietic cells activate alloreactive CD8+ T lymphocytes in vitro and in vivo. We also show that vascularized cardiac allografts are acutely rejected via CD8+ direct allorecognition even if the alloantigen is not presented by hematopoietic professional antigen-presenting cells. Because activation of alloreactive CD8+ T cells by donor-type non-hematopoietic cells can continue for the life of the allograft, these findings present a new clinically relevant mechanism of allorecognition and should be taken into consideration when developing strategies to prevent allograft vasculopathy or to induce tolerance.