Allergic conjunctivitis exacerbates corneal allograft rejection by activating Th1 and th2 alloimmune responses

Case Report: A Case of Acute Cellular Rejection Due to Atopic Dermatitis Exacerbation 3 Years After Heart Transplantation

Background

Little evidence has been presented about the association between previous atopic/allergic disease and graft rejection after solid organ transplantation. Thus, we present a case wherein acute cellular rejection (ACR) after heart transplantation (HTx) was noted along with exacerbation of atopic disease.

Case Summary

A 32-year-old man was admitted at our hospital for regular monitoring of graft rejection. He had undergone heart transplant 3 years prior due to dilated cardiomyopathy. Echocardiogram revealed good biventricular function, and no abnormal findings were found in blood sampling tests. However, biopsy showed moderate ACR [Grade 2R(ISHLT 2004)/3A(ISHLT 1990)], which required twice-repeated steroid pulses with intensified immunosuppression. Meanwhile, his atopic dermatitis, which was diagnosed before having heart failure, was getting worse for the past 6 months. The exacerbation of atopic dermatitis was presumed to be related to the development of the intractable cellular rejection.

Discussion

This case suggested the association of atopic disease and graft rejection after HTx. We examined 76 patients from a cohort of previous studies who underwent HTx at our hospital, which suggested that patients with atopic/allergic disorders such as atopic dermatitis and asthma tended to have a significantly higher frequency of moderate rejection than non-allergic patients. (p = 0.012; Fisher’s exact test). Our case also suggests that exacerbation of atopic dermatitis might cause graft rejection of the transplanted organ, so that it is important to carefully evaluate the risk of graft rejection if there is a previous history of atopic/allergic disease.

Animal models of high-risk corneal transplantation: A comprehensive review

Over the past century, corneal transplantation has become the most commonly performed allogeneic solid tissue transplantation. Although more than 80% of the corneal transplantations have favorable outcomes, immune-mediated rejection continues to be the major cause of failure in well over 50% of graft recipients that have inflamed and vascularized host beds. Over the past two decades, the progress in our understanding of the immunological pathways that mediate graft rejection has aided in the development of novel therapeutic strategies. In order to successfully test the efficacy of these interventions, it is essential to model the immunological processes occurring as a consequence of corneal transplantation. Herein, we have comprehensively reviewed the established animal models used for replicating the immunopathological processes causing graft rejection in high-risk corneal transplantation settings. We have also discussed the practical and technical differences, as well as biological and immunological variations in different animal models.

Endogenous Toll-like Receptor 2 Modulates Th1/Treg-Promoting Dendritic Cells in Mice Corneal Transplantation Model

PURPOSE

Endogenous toll-like receptor (TLR) 2 is linked to allograft rejection in corneal transplantation. TLR2 also could modulate dendritic cell (DC) phenotype, resulting in T cell polarization. Thus, we investigated the role of endogenous TLR2 on DC development and T cell polarization during corneal rejection.

MATERIALS AND METHODS

Corneas of BALB/c mice were transplanted into the eyes of C57BL/6 wild-type (WT) recipients and TLR2^-/- (KO) recipients. Graft survival and TLR2 mRNA expression were assessed. At day 14 after transplantation, to study endogenous TLR2 effects on DC development and function, surface expression of MHC classⅡ (MHCⅡ), CD86, CD80 and CD40 in ipsilateral cervical draining lymph nodes (DLNs) is measured by flow cytometry, and DC phenotype in corneas is detected by immunofluorescence. The levels of IL-12, IL-10 and IL-4 in corneas were measured by real time-qPCR (RT-qPCR). The ability of DCs to stimulate T cell polarization was assessed by IFN-γ expressions via RT-qPCR and immunohistochemistry.

RESULTS

TLR2 mRNA expression in corneas was peaked at day 14 post-transplantation in WT group. KO group improved corneal allograft survival compared to the WT group. In addition, the KO group decreased expression of CD86, CD80 and CD40 on DCs compared to the WT group. There was no difference in MHCⅡ expression in two groups. The CD11c⁺MHCⅡ⁺CD40^high DCs could not be detected in corneas of the KO group. Moreover, the KO group decreased IL-12 (Th1-promoting cytokines) mRNA expression and increasing IL-10 (Treg-promoting cytokines) mRNA expression compared to the WT group. IL-4 (Th2-promoting cytokines) mRNA expression gained no difference between the two groups. The IFN-γ (Th1 cytokines) expression was significantly decreased in the KO group compared to the WT group.

CONCLUSIONS

Endogenous TLR2 may contribute to allogeneic corneal rejection via Th1 immunity by activating Th1-promoting DCs and suppressing Treg-promoting DCs.

Immune reactions after modern lamellar (DALK, DSAEK, DMEK) versus conventional penetrating corneal transplantation

In the past decade, novel lamellar keratoplasty techniques such as Deep Anterior Lamellar Keratoplasty (DALK) for anterior keratoplasty and Descemet stripping automated endothelial keratoplasty (DSAEK)/Descemet membrane endothelial keratoplasty (DMEK) for posterior keratoplasty have been developed. DALK eliminates the possibility of endothelial allograft rejection, which is the main reason for graft failure after penetrating keratoplasty (PK). Compared to PK, the risk of endothelial graft rejection is significantly reduced after DSAEK/DMEK. Thus, with modern lamellar techniques, the clinical problem of endothelial graft rejection seems to be nearly solved in the low-risk situation. However, even with lamellar grafts there are epithelial, subepithelial and stromal immune reactions in DALK and endothelial immune reactions in DSAEK/DMEK, and not all keratoplasties can be performed in a lamellar fashion. Therefore, endothelial graft rejection in PK is still highly relevant, especially in the "high-risk" setting, where the cornea's (lymph)angiogenic and immune privilege is lost due to severe inflammation and pathological neovascularization. For these eyes, currently available treatment options are still unsatisfactory. In this review, we will describe currently used keratoplasty techniques, namely PK, DALK, DSAEK, and DMEK. We will summarize their indications, provide surgical descriptions, and comment on their complications and outcomes. Furthermore, we will give an overview on corneal transplant immunology. A specific focus will be placed on endothelial graft rejection and we will report on its incidence, clinical presentation, and current/future treatment and prevention options. Finally, we will speculate how the field of keratoplasty and prevention of corneal allograft rejection will develop in the future.

Immune privilege in corneal transplantation

Corneal transplantation is the most successful solid organ transplantation performed in humans. The extraordinary success of orthotopic corneal allografts, in both humans and experimental animals, is related to the phenomenon of "immune privilege". Inflammation is self-regulated to preserve ocular functions because the eye has immune privilege. At present, three major mechanisms are considered to provide immune privilege in corneal transplantation: 1) anatomical, cellular, and molecular barriers in the cornea; 2) tolerance related to anterior chamber-associated immune deviation and regulatory T cells; and 3) an immunosuppressive intraocular microenvironment. This review describes the mechanisms of immune privilege that have been elucidated from animal models of ocular inflammation, especially those involving corneal transplantation, and its relevance for the clinic. An update on molecular, cellular, and neural interactions in local and systemic immune regulation is provided. Therapeutic strategies for restoring immune privilege are also discussed.

AICAR prolongs corneal allograft survival via the AMPK-mTOR signaling pathway in mice

Immune rejection is a critical complication that results in the graft failure after corneal transplantation. Thus, there remains a need for new therapies for allograft rejection. AICAR (aminoimidazole-4-carboxamide ribonucleoside) is an, as adenosine monophosphate-activated protein kinase (AMPK) activator and a purine nucleoside with a wide range of metabolic effects, including activation of AMPK. More recently, it was reported that it is possible to inhibiting organs rejection and prolong the graft survival time in various models of organ transplantation. In this study, we systematically evaluated the efficacy of AICAR as a treatment modality for inhibiting allograft rejection in a mouse model of corneal transplantation. We found that AICAR significantly suppressed the opacity, edema, and vascularization of the graft, resulting in prolonged corneal allograft survival. AICAR treatment also significantly decreased central corneal thickness. Moreover, the AICAR-treated group showed decreased expression of IB4 and VEGF as compared to the control group. In addition, the mRNA expression of T helper 1 cytokines (IL-2, INF-γ, and TNF-α) was suppressed, and the expression of T helper 2 cytokines (IL-4, IL-5, and IL-13) was elevated by AICAR. Furthermore, the western blotting results revealed that AICAR stimulated AMPK activation and inhibited angiogenesis and inflammation possibly by subsequently suppressing mTOR phosphorylation. By contrast, the AMPK inhibitor Compound C (also called dorsomorphin) had the opposite effect. Our results showed that Compound C blocked AMPK-mTOR signaling and promoted the angiogenesis and inflammation, thus compromising the graft survival. These results suggest that AICAR may be a potential option for inhibiting the corneal graft rejection and for prolonging the graft survival.

Immune Privilege and Eye-Derived T-Regulatory Cells

Certain cellular components of the eye, such as neural retina, are unable to regenerate and replicate after destructive inflammation. Ocular immune privilege provides the eye with immune protection against intraocular inflammation in order to minimize the risk to vision integrity. The eye and immune system use strategies to maintain the ocular immune privilege by regulating the innate and adaptive immune response, which includes immunological ignorance, peripheral tolerance to eye-derived antigens, and intraocular immunosuppressive microenvironment. In this review, we summarize current knowledge regarding the molecular mechanism responsible for the development and maintenance of ocular immune privilege via regulatory T cells (Tregs), which are generated by the anterior chamber-associated immune deviation (ACAID), and ocular resident cells including corneal endothelial (CE) cells, ocular pigment epithelial (PE) cells, and aqueous humor. Furthermore, we examined the therapeutic potential of Tregs generated by RPE cells that express transforming growth factor beta (TGF-β), cytotoxic T lymphocyte-associated antigen-2 alpha (CTLA-2α), and retinoic acid for autoimmune uveoretinitis and evaluated a new strategy using human RPE-induced Tregs for clinical application in inflammatory ocular disease. We believe that a better understanding of the ocular immune privilege associated with Tregs might offer a new approach with regard to therapeutic interventions for ocular autoimmunity.

Ocular surgical models for immune and angiogenic responses

Corneal transplantation serves as a reproducible and simple surgical model to study mechanisms regulating immunity and angiogenesis. The simplicity of the model allows for systematic analysis of different mechanisms involved in immune and angiogenic privilege and their failures. This protocol describes how to induce neovessels and inflammation in an actively regulated avascular and immune-privileged site. This involves placing intra-stromal corneal sutures for two weeks, disrupting the privileges, and performing corneal transplantation subsequently. Privileged and non-privileged recipient responses to donor cornea can be compared to identify key immunological mechanisms that underlie angiogenesis and graft rejection. This protocol can also be adapted to the growing repertoire of genetic models available in the mouse, and is a valuable tool to elucidate molecular mechanisms mediating acceptance or failure of corneal graft. The model could be used to assess the potential of therapeutic molecules to enhance graft survival in vivo.

Protection by LPS-induced inhibitory CD11b(+) cells on corneal allograft

OBJECTIVE

It is widely reported that CD11b(+)Gr1+ myeloid-derived suppressor cells can cause allograft tolerance in mice and human, however, little is known on the therapy role in chronic transplantation rejection. In this paper, their role in corneal transplantation was studied for the first time.

METHOD

Inhibitory CD11b(+) cells were obtained by murine LPS-induced septic model. Phenotype, endocytosis, antigen presenting ability, and T cell suppression assays were performed by flow cytometry analysis. The suppressive ability in vivo was analyzed by targeting allogeneic corneal transplantation.

RESULTS

LPS was intraperitoneally injected into C57BL/6 mice, the percentage of CD11b(+) Gr1+ cells was increased in mice spleen, blood, and bone marrow, respectively. Compared with control mice, Ly6C, TLR2, and MHC-11 expression were higher in LPS treated mice. CD11b(+) Gr1+ cells could inhibit allogenic corneal reaction in vivo after adoptive transfer, in consistent with an observation of inhibition effect on the antigen presenting cells (APCs) and CD4+ T cells proliferation in vitro.

CONCLUSION

CD11b(+) cells induced by LPS could function as inhibitory APCs, suppress CD4+ T cells proliferation and improve corneal allograft survival. Predictly, its application for cells transfer therapy in clinic in the further.

Immunology of Corneal Allografts: Insights from Animal Models

Corneal transplantation stands alone as the most common and successful form of solid organ transplantation. Even though HLA matching and systemic antirejection drugs are not routinely used, 90% of the first time corneal allografts will succeed. By contrast, all other major categories of organ transplantation require HLA matching and the use of systemically administered immunosuppressive drugs. This remarkable success of corneal transplants under these conditions is an example of "immune privilege" and is the primary reason for the extraordinary success of corneal transplantation. A number of dogmas have emerged over the past century to explain immune privilege and the immunobiology of corneal transplantation. Many of these dogmas have been based largely on inferences from clinical observations on keratoplasty patients. The past 30 years have witnessed a wealth of rodent studies on corneal transplantation that have tested hypotheses and dogmas that originated from clinical observations on penetrating keratoplasty patients. Rodent models allow the application of highly sophisticated genetic and immunological tools for testing these hypotheses in a controlled environment and with experiments designed prospectively. These studies have validated some of the widely held assumptions based on clinical observations and in other cases, previous dogmas have been replaced with new insights that could only come from prospective studies performed under highly controlled conditions. This review highlights some of the key dogmas and these widely held assumptions that have been scrutinized through the use of rodent models of penetrating keratoplasty. This review also makes note of new immunological principles of corneal immunology that have emerged from rodent studies on corneal transplantation that most likely would not have been revealed in studies on corneal transplantation patients.

IFN-γ blocks CD4+CD25+ Tregs and abolishes immune privilege of minor histocompatibility mismatched corneal allografts

Th1 CD4+ cells are believed to be the primary mediators of corneal allograft rejection. However, rejection of fully allogeneic C57BL/6 corneal allografts soared from 50% to 90% in both interferon-gamma (IFN-γ)(-/-) and anti-IFN-γ-treated BALB/c mice. In contrast, similar deficits in IFN-γ in BALB/c hosts enhanced immune privilege of BALB.B (minor histocompatibility [minor H] antigen-matched, major histocompatibility complex [MHC]-mismatched) and NZB (MHC-matched, minor H antigen-mismatched) corneal allografts-decreasing rejection from 80% to ~20%. This effect of IFN-γ was independent of CD4+ T cell lineage commitment as both anti-IFN-γ-treated acceptor and rejector mice displayed a Th2 cytokine profile. The presence of IFN-γ prevented the generation of alloantigen-specific CD4+CD25+ T regulatory cells (Tregs) in hosts receiving either MHC only mismatched BALB.B or minor only histocompatibility (minor H)-mismatched NZB corneal allografts. Tregs in these hosts promoted corneal allograft survival by suppressing Th2 effector cells. By contrast, IFN-γ was necessary for the generation of CD4+CD25+ Tregs that prevented rejection of fully allogeneic C57BL/6 corneal allografts in BALB/c hosts. These findings suggest that MHC-matching in combination with blockade of IFN-γ holds promise as a means of enhancing corneal allograft survival.

Corneal transplantation and immune privilege

Corneal transplants have been successfully performed in human subjects for over 100 years and enjoy an immune privilege that is unrivaled in the field of transplantation. Immune privilege is defined as the reduced incidence and tempo in the immune rejection of corneal allografts compared to other categories of organ allografts performed under the same conditions. Skin allografts transplanted across various MHC or minor histocompatibility barriers undergo rejection in approximately 100% of the hosts. By contrast, orthotopic corneal allografts experience long-term survival in 50% to >90% of the hosts, depending on the histocompatibility barriers that confront the host. The capacity of corneal allografts to evade immune rejection is attributable to multiple anatomical, physiological and immunoregulatory conditions that conspire to prevent the induction and expression of alloimmunity.

Allergic conjunctivitis: a comprehensive review of the literature

Ocular allergy represents one of the most common conditions encountered by allergists and ophthalmologists. Allergic conjunctivitis is often underdiagnosed and consequently undertreated. Basic and clinical research has provided a better understanding of the cells, mediators, and immunologic events, which occur in ocular allergy. New pharmacological agents have improved the efficacy and safety of ocular allergy treatment. An understanding of the immunologic mechanisms, clinical features, differential diagnosis, and treatment of ocular allergy may be useful to all specialists who deal with these patients. The purpose of this review is to systematically review literature underlining all the forms classified as ocular allergy: seasonal allergic conjunctivitis, perennial allergic conjunctivitis, vernal keratoconjunctivitis, atopic keratocongiuntivitis, contact allergy, and giant papillary conjunctivitis.

New twists to an old story: novel concepts in the pathogenesis of allergic eye disease

The prevalence of allergy is rising globally at a very significant rate, which is currently at 20-40% of individuals in westernized nations. In the eye, allergic conditions can take on the acute form such as in seasonal and perennial allergic conjunctivitis, or a more severe and debilitating chronic form such as in vernal and atopic keratoconjunctivitis. Indeed, some key aspects of allergic eye disease pathophysiology are understood, such as the role of mast cells in the acute allergic reaction, and the contribution of eosinophils in late-onset and chronic allergy. However, recent developments in animal models and clinical studies have uncovered new and important roles for previously underappreciated players, including chemokine receptors on ocular surface dendritic cells such as CCR7, the contribution of conjunctival epithelium to immunity, histamine and leukotriene receptors on conjunctival goblet cells and a role for mast cells in late-onset manifestations. Furthermore, recent work in animal models has delineated the contribution of IL-4 in the increased incidence of corneal graft rejection in hosts with allergic conjunctivitis. Recent studies such as these mean that conventional paradigms and concepts should be revisited. The aim of this review is to highlight some of the most recent advances and insights on newly appreciated players in the pathogenesis of allergic eye disease.

Gamma-irradiation reduces the allogenicity of donor corneas

PURPOSE

To evaluate the utility and allogenicity of gamma-irradiated corneal allografts.

METHODS

Corneal buttons were harvested from C57BL/6 mice and decellularized with gamma irradiation. Cell viability was assessed using TUNEL and viability/cytotoxicity assays. Orthotopic penetrating keratoplasty was performed using irradiated or nonirradiated (freshly excised) C57BL/6 donor grafts and BALB/c or C57BL/6 recipients. Graft opacity was assessed over an 8-week period and graft survival was evaluated using Kaplan-Meier survival curves. Mixed-lymphocyte reactions and delayed-type hypersensitivity assays were performed to evaluate T-cell alloreactivity. Real-time PCR was used to investigate the corneal expression of potentially pathogenic T-helper 1, 2, and 17 cell-associated cytokines.

RESULTS

Corneal cells were devitalized by gamma irradiation as evidenced by widespread cellular apoptosis and plasma membrane disruption. Nonirradiated allograft and isograft rates of survival were superior to irradiated allograft and isograft rates of survival (P < 0.001). Mixed lymphocyte reactions demonstrated that T-cells from irradiated allograft recipients did not exhibit a secondary alloimmune response (P < 0.001). Delayed-type hypersensitivity assays demonstrated that irradiated allografts did not elicit an alloreactive delayed-type hypersensitivity response in graft recipients (P ≤ 0.01). The corneal expression of T-helper 1, 2, and 17 cell-associated cytokines was significantly lower in failed irradiated allografts than rejected nonirradiated allografts (P ≤ 0.001).

CONCLUSIONS

Gamma-irradiated corneas failed to remain optically clear following murine penetrating keratoplasty; however, gamma irradiation reduced the allogenicity of these corneas, potentially supporting their use in procedures such as anterior lamellar keratoplasty or keratoprosthesis implantation.

Recent patents and emerging therapeutics in the treatment of allergic conjunctivitis

Ocular allergy is an inflammatory response of the conjunctival mucosa that also affects the cornea and eyelids. Allergic conjunctivitis includes seasonal allergic conjunctivitis (SAC), perennial allergic conjunctivitis (PAC), vernal keratoconjunctivitis (VKC), atopic keratoconjunctivitis (AKC) and giant papillary conjunctivitis (GPC). In general, allergic conditions involve mast cell degranulation that leads to release of inflammatory mediators and activation of enzymatic cascades generating pro-inflammatory mediators. In chronic ocular inflammatory disorders associated with mast cell activation such as VKC and AKC constant inflammatory response is observed due to predominance of inflammatory mediators such as eosinophils and Th2-generated cytokines. Antihistamines, mast-cell stabilizers, nonsteroidal anti-inflammatory agents, corticosteroids and immunomodulatory agents are commonly indicated for the treatment of acute and chronic allergic conjunctivitis. In recent years newer drug molecules have been introduced in the treatment of allergic conjunctivitis. This article reviews recent patents and emerging therapeutics in the treatment of allergic conjunctivitis.

Eosinophilic/T-cell chorionic vasculitis: a clinicopathologic and immunohistochemical study of 51 cases

We report 51 placentas diagnosed with eosinophilic/T-cell chorionic vasculitis (E/TCV), an unusual form of chorionic vasculitis characterized by an infiltrate composed predominantly of CD3+ T cells and eosinophils. The placentas were all 3rd trimester, with 48 (94.1%) being term. Forty-seven (92.2%) were singleton placentas, and the remaining 4 were twins. The E/TCV was limited to 1 chorionic surface vessel in 40 (78.4%) and involved 50% or less of the vessel circumference in 30 (58.8%) placentas. The inflammation faced the intervillous space in 12 (23.5%) and the amniotic cavity in 8 (15.7%) and had no distinct predominant direction in the remaining 31 (60.8%) placentas. Twelve (25.5%) placentas showed mural thrombi or intramural fibrin in association with the E/TCV. One hundred six term singleton placentas were selected as the control group, and the 47 singleton placentas with E/TCV made up the study group for comparison of demographic and histopathologic features. Villitis of unknown etiology was identified more frequently in study group placentas (20 [42.6%]) compared with control group placentas (14 [13.2%]) (P < 0.001). Vascular changes of fetal vascular thrombo-occlusive disease were identified away from the E/TCV more frequently in study group placentas (8 [17.0%]) compared with control group placentas (4 [3.8%]) (P  =  0.008). There were no significant differences in the frequencies of other placental lesions studied, including acute inflammatory lesions and lesions related to maternal underperfusion. There were no significant differences in maternal age, race, parity, birth weight, allergy history, blood type, or medication use.

High-risk corneal allografts and why they lose their immune privilege

PURPOSE OF REVIEW

Corneal allografts are routinely performed without HLA typing or systemic immunosuppressive drugs. However, certain conditions create high risks for immune rejection. This review discusses recent insights into the mechanisms that rob the corneal allograft of its immune privilege.

RECENT FINDINGS

Studies in mice have revealed that stimuli that induce new blood vessel growth in the cornea also elicit proliferation of lymph vessels. Lymph vessels facilitate migration of antigen-presenting cells to regional lymph nodes in which they induce alloimmune responses. The presence of blood vessels in the corneal graft bed creates a unique chemokine milieu that stimulates recruitment of sensitized lymphocytes into the corneal allograft. Other data indicate that although corneal allograft survival is closely associated with Foxp3 expression in CD4+CD25+Foxp3+ T regulatory cells (Tregs), reduced expression of Foxp3 in Tregs creates a high risk for graft rejection. Recent evidence indicates that allergic diseases have a profound impact on the immune response and produce a dramatic increase in corneal allograft rejection.

SUMMARY

Understanding the underlying mechanisms that create 'high-risk' hosts may provide important therapeutic targets for restoring immune privilege of corneal allografts and enhancing their survival.

Paradigm shifts in the role of CD4+ T cells in keratoplasty

Corneal transplantation is the oldest, most common, and arguably the most successful form of organ transplantation. In uncomplicated first-time cases, corneal allografts enjoy a success rate of up to 90% even though the transplants are performed without HLA matching or the use of systemic immunosuppressive drugs. In rodents, corneal allografts transplanted across entire MHC and multiple minor histocompatibility barriers enjoy long-term survival in >50% of the hosts, while skin grafts invariably undergo immune rejection. These observations are the basis for "immune privilege" of corneal transplants. In spite of this immune privilege, immune rejection can occur and remains the leading cause of corneal graft failure. Rodent models of penetrating keratoplasty have facilitated studies that have challenged, and in some cases, refuted prevailing dogmas. The long-held belief that CD4+ T helper 1 (Th1) cells were the sole mediators of corneal allograft rejection has fallen to the wayside based on studies in interferon-gamma (IFN-γ)⁻(/)⁻ mice. The dogma that biasing the alloimmune response down a Th2 pathway would enhance graft survival has also been disproven, and in fact, compelling evidence indicates that Th2-based immune rejection of corneal allografts is swifter and more intense than Th1-based rejection. Animal studies have also pre-empted emerging dogmas including the hypothesis that Th17 cells play a crucial role in allograft rejection. Instead, IL-17A appears to be necessary for corneal allograft survival. Finally, IFN-γ, and IL-17A, which were normally viewed as proinflammatory, exert the opposite effect in the context of corneal transplantation and are necessary for corneal allograft survival.