AUTOPHAGY IN THE EYE: FROM PHYSIOLOGY TO PATHOPHYSOLOGY

Autophagy is a catabolic self-degradative pathway that promotes the degradation and recycling of intracellular material through the lysosomal compartment. Although first believed to function in conditions of nutritional stress, autophagy is emerging as a critical cellular pathway, involved in a variety of physiological and pathophysiological processes. Autophagy dysregulation is associated with an increasing number of diseases, including ocular diseases. On one hand, mutations in autophagy-related genes have been linked to cataracts, glaucoma, and corneal dystrophy; on the other hand, alterations in autophagy and lysosomal pathways are a common finding in essentially all diseases of the eye. Moreover, LC3-associated phagocytosis, a form of non-canonical autophagy, is critical in promoting visual cycle function. This review collects the latest understanding of autophagy in the context of the eye. We will review and discuss the respective roles of autophagy in the physiology and/or pathophysiology of each of the ocular tissues, its diurnal/circadian variation, as well as its involvement in diseases of the eye.

A Study of Superhomogenization Applied to PHWR Lattices

To reduce computational expenses, full-core production-type neutronics calculations are customarily performed using a simplified core-model whereby large regions of the core, called nodes, are assumed to be homogeneous. The process of generating the few-group homogenized-node macroscopic cross sections is called lattice homogenization. The simplest homogenization method is standard homogenization (SH) and full-core models based on it do not usually reproduce heterogeneous-core calculations too closely. To improve agreement between node-homogenized core results and heterogeneous-core results, advanced homogenization techniques are used. Such techniques tend to use additional parameters besides homogenized macroscopic cross sections. Superhomogenization (SPH) is an advanced lattice homogenization method, which has been developed initially for light-water-reactor (LWR) lattices whereby fuel elements are arranged in a rectangular array. It has the advantage of not requiring any modification to the full-core diffusion code for its implementation. For LWRs, SPH establishes neutronic equivalence between detailed-geometry heterogeneous fuel-pin cells and homogenized fuel-pin cells by adjusting homogenized multigroup macroscopic cross sections and diffusion coefficients. This work investigates the possible use of the SPH methodology for pressurized heavy-water reactor (PHWR) lattices whose fuel pins are arranged in concentric rings rather than in a rectangular array. Results for single-node (SN) as well as multinode (MN) lattice-calculation models are presented. Results show that, with proper region definition, the SPH methodology can be used for PHWR lattices but that improvement in homogenization accuracy is only marginal compared with SH when comparing results for the same type of lattice model (SN or MN).

RUBCN/rubicon and EGFR regulate lysosomal degradative processes in the retinal pigment epithelium (RPE) of the eye

Macroautophagy/autophagy is an intracellular stress survival and recycling system whereas phagocytosis internalizes material from the extracellular milieu; yet, both pathways utilize lysosomes for cargo degradation. Whereas autophagy occurs in all cells, phagocytosis is performed by cell types such as macrophages and the retinal pigment epithelial (RPE) cells of the eye where it is supported by the noncanonical autophagy process termed LC3-associated phagocytosis (LAP). Autophagy and LAP are distinct pathways that use many of the same mediators and must compete for cellular resources, suggesting that cells may regulate both processes under homeostatic and stress conditions. Our data reveal that RPE cells promote LAP through the expression of RUBCN/Rubicon (RUN domain and cysteine-rich domain containing Beclin 1-interacting protein) and suppress autophagy through the activation of EGFR (epidermal growth factor receptor). In the morning when photoreceptor outer segments (POS) phagocytosis and LAP are highest, RUBCN expression is increased. At the same time, outer segment phagocytosis activates the EGFR resulting in MTOR (mechanistic target of rapamycin [serine/threonine kinase]) stimulation, the accumulation of SQSTM1/p62, and the phosphorylation of BECN1 (Beclin 1, autophagy related) on an inhibitory residue thereby suppressing autophagy. Silencing Rubcn, preventing EGFR activity or directly inducing autophagy in RPE cells by starvation inhibits phagocytic degradation of POS. Thus, RPE cells regulate lysosomal pathways during the critical period of POS phagocytosis to support retinal homeostasis.

Inhibiting autophagy reduces retinal degeneration caused by protein misfolding

Mutations in the genes necessary for the structure and function of vertebrate photoreceptor cells are associated with multiple forms of inherited retinal degeneration. Mutations in the gene encoding RHO (rhodopsin) are a common cause of autosomal dominant retinitis pigmentosa (adRP), with the Pro23His variant of RHO resulting in a misfolded protein that activates endoplasmic reticulum stress and the unfolded protein response. Stimulating macroautophagy/autophagy has been proposed as a strategy for clearing misfolded RHO and reducing photoreceptor death. We found that retinas from mice heterozygous for the gene encoding the RHOP23H variant (hereafter called P23H) exhibited elevated levels of autophagy flux, and that pharmacological stimulation of autophagy accelerated retinal degeneration. In contrast, reducing autophagy flux pharmacologically or by rod-specific deletion of the autophagy-activating gene Atg5, improved photoreceptor structure and function. Furthermore, proteasome levels and activity were reduced in the P23H retina, and increased when Atg5 was deleted. Our findings suggest that autophagy contributes to photoreceptor cell death in P23H mice, and that decreasing autophagy shifts the degradation of misfolded RHO protein to the proteasome and is protective. These observations suggest that modulating the flux of misfolded proteins from autophagy to the proteasome may represent an important therapeutic strategy for reducing proteotoxicity in adRP and other diseases caused by protein folding defects.

Autophagy-mediated catabolism of visual transduction proteins prevents retinal degeneration

Autophagy is a lysosomal degradation pathway critical to preventing the accumulation of cytotoxic proteins. Deletion of the essential autophagy gene Atg5 from the rod photoreceptors of the retina (atg5^Δrod mouse) results in the accumulation of the phototransduction protein transducin and the degeneration of these neurons. The purpose of this study is to test the hypothesis that autophagic degradation of visual transduction proteins prevents retinal degeneration. Targeted deletion of both Gnat1 (a gene encoding the α subunit of the heterotrimeric G-protein transducin) and Atg5 in the rod photoreceptors resulted in a significantly decreased rate of rod cell degeneration as compared to the atg5^Δrod mouse retina, and considerable preservation of photoreceptors. Supporting this we used a novel technique to immunoprecipitate green fluorescent protein (GFP)-tagged autophagosomes from the retinas of the GFP-LC3 mice and demonstrated that the visual transduction proteins transducin and ARR/arrestin are associated with autophagosome-specific proteins. Altogether, this study shows that degradation of phototransduction proteins by autophagy is necessary to prevent retinal degeneration. In addition, we demonstrate a simple and easily reproducible immunoisolation technique for enrichment of autophagosomes from the GFP-LC3 mouse retina, providing a novel application to the study of autophagosome contents across different organs and specific cell types in vivo.

Autophagy supports color vision

Cones comprise only a small portion of the photoreceptors in mammalian retinas. However, cones are vital for color vision and visual perception, and their loss severely diminishes the quality of life for patients with retinal degenerative diseases. Cones function in bright light and have higher demand for energy than rods; yet, the mechanisms that support the energy requirements of cones are poorly understood. One such pathway that potentially could sustain cones under basal and stress conditions is macroautophagy. We addressed the role of macroautophagy in cones by examining how the genetic block of this pathway affects the structural integrity, survival, and function of these neurons. We found that macroautophagy was not detectable in cones under normal conditions but was readily observed following 24 h of fasting. Consistent with this, starvation induced phosphorylation of AMPK specifically in cones indicating cellular starvation. Inhibiting macroautophagy in cones by deleting the essential macroautophagy gene Atg5 led to reduced cone function following starvation suggesting that cones are sensitive to systemic changes in nutrients and activate macroautophagy to maintain their function. ATG5-deficiency rendered cones susceptible to light-induced damage and caused accumulation of damaged mitochondria in the inner segments, shortening of the outer segments, and degeneration of all cone types, revealing the importance of mitophagy in supporting cone metabolic needs. Our results demonstrate that macroautophagy supports the function and long-term survival of cones providing for their unique metabolic requirements and resistance to stress. Targeting macroautophagy has the potential to preserve cone-mediated vision during retinal degenerative diseases.

Impaired autophagy in macrophages promotes inflammatory eye disease

Autophagy is critical for maintaining cellular homeostasis. Organs such as the eye and brain are immunologically privileged. Here, we demonstrate that autophagy is essential for maintaining ocular immune privilege. Deletion of multiple autophagy genes in macrophages leads to an inflammation-mediated eye disease called uveitis that can cause blindness. Loss of autophagy activates inflammasome-mediated IL1B secretion that increases disease severity. Inhibition of caspase activity by gene deletion or pharmacological means completely reverses the disease phenotype. Of interest, experimental uveitis was also increased in a model of Crohn disease, a systemic autoimmune disease in which patients often develop uveitis, offering a potential mechanistic link between macrophage autophagy and systemic disease. These findings directly implicate the homeostatic process of autophagy in blinding eye disease and identify novel pathways for therapeutic intervention in uveitis.

IL-18 Immunotherapy for Neovascular AMD: Tolerability and Efficacy in Nonhuman Primates

PURPOSE

Age-related macular degeneration is the most common form of central retinal blindness in the elderly. Of the two end stages of disease, neovascular AMD-although the minority form-is the most severe. Current therapies are highly successful at controlling progression of neovascular lesions; however, a significant number of patients remain refractory to treatment and the development of alternative and additive therapies to anti-VEGFs is essential.

METHODS

In order to address the translational potential of interleukin (IL)-18 for use in neovascular AMD, we initiated a nonhuman primate tolerability and efficacy study for the use of intravitreally (IVT) administered clinical grade human IL-18 (SB-485232). Cynomolgus monkeys were injected IVT with increasing doses of human IL-18 (two each at 1000, 3000, and 10,000 ng per eye). In tandem, 21 monkeys were administered nine laser burns in each eye prior to receiving IL-18 as an IVT injection at a range of doses. Fundus fluorescein angiography (FFA) was performed on days 8, 15, and 22 post injection and the development of neovascular lesions was assessed.

RESULTS

We show intravitreal, mature, recombinant human IL-18 is safe and can reduce choroidal neovascular lesion development in cynomolgus monkeys.

CONCLUSIONS

Based on our data comparing human IL-18 to current anti-VEGF-based therapy, clinical deployment of IL-18 for neovascular AMD has the potential to lead to a new adjuvant immunotherapy-based treatment for this severe form of central blindness.

Autophagy supports survival and phototransduction protein levels in rod photoreceptors

Damage and loss of the postmitotic photoreceptors is a leading cause of blindness in many diseases of the eye. Although the mechanisms of photoreceptor death have been extensively studied, few studies have addressed mechanisms that help sustain these non-replicating neurons for the life of an organism. Autophagy is an intracellular pathway where cytoplasmic constituents are delivered to the lysosomal pathway for degradation. It is not only a major pathway activated in response to cellular stress, but is also important for cytoplasmic turnover and to supply the structural and energy needs of cells. We examined the importance of autophagy in photoreceptors by deleting the essential autophagy gene Atg5 specifically in rods. Loss of autophagy led to progressive degeneration of rod photoreceptors beginning at 8 weeks of age such that by 44 weeks few rods remained. Cone photoreceptor numbers were only slightly diminished following rod degeneration but their function was significantly decreased. Rod cell death was apoptotic but was not dependent on daily light exposure or accelerated by intense light. Although the light-regulated translocation of the phototransduction proteins arrestin and transducin were unaffected in rods lacking autophagy, Atg5-deficient rods accumulated transducin-α as they degenerated suggesting autophagy might regulate the level of this protein. This was confirmed when the light-induced decrease in transducin was abolished in Atg5-deficient rods and the inhibition of autophagy in retinal explants cultures prevented its degradation. These results demonstrate that basal autophagy is essential to the long-term health of rod photoreceptors and a critical process for maintaining optimal levels of the phototransduction protein transducin-α. As the lack of autophagy is associated with retinal degeneration and altered phototransduction protein degradation in the absence of harmful gene products, this process may be a viable therapeutic target where rod cell loss is the primary pathologic event.

An immunogenic peptide in the A-box of HMGB1 protein reverses apoptosis-induced tolerance through RAGE receptor

Apoptotic cells trigger immune tolerance in engulfing phagocytes. This poorly understood process is believed to contribute to the severe immunosuppression and increased susceptibility to nosocomial infections observed in critically ill sepsis patients. Extracellular high mobility group box 1 (HMGB1) is an important mediator of both sepsis lethality and the induction of immune tolerance by apoptotic cells. We have found that HMGB1 is sensitive to processing by caspase-1, resulting in the production of a fragment within its N-terminal DNA-binding domain (the A-box) that signals through the receptor for advanced glycation end products (RAGE) to reverse apoptosis-induced tolerance. In a two-hit mouse model of sepsis, we show that tolerance to a secondary infection and its associated mortality were effectively reversed by active immunization with dendritic cells treated with HMGB1 or the A-box fragment, but not a noncleavable form of HMGB1. These findings represent a novel link between caspase-1 and HMGB1, with potential therapeutic implications in infectious and inflammatory diseases.

Autophagy and phagocytosis converge for better vision

The retinal pigment epithelium (RPE) is a single layer of nonregenerating cells essential to homeostasis in the retina and the preservation of vision. While the RPE perform a number of important functions, 2 essential processes are phagocytosis, which removes the most distal tips of the photoreceptors to support disk renewal, and the visual cycle, which maintains the supply of chromophore for regeneration of photo-bleached visual pigments. We recently reported that these processes are linked by a noncanonical form of autophagy termed LC3-associated phagocytosis (LAP) in which components of the autophagy pathway are co-opted by phagocytosis to recover vitamin A in support of optimal vision. Here we summarize these findings.

Noncanonical autophagy promotes the visual cycle

Phagocytosis and degradation of photoreceptor outer segments (POS) by retinal pigment epithelium (RPE) is fundamental to vision. Autophagy is also responsible for bulk degradation of cellular components, but its role in POS degradation is not well understood. We report that the morning burst of RPE phagocytosis coincided with the enzymatic conversion of autophagy protein LC3 to its lipidated form. LC3 associated with single-membrane phagosomes containing engulfed POS in an Atg5-dependent manner that required Beclin1, but not the autophagy preinitiation complex. The importance of this process was verified in mice with Atg5-deficient RPE cells that showed evidence of disrupted lysosomal processing. These mice also exhibited decreased photoreceptor responses to light stimuli and decreased chromophore levels that were restored with exogenous retinoid supplementation. These results establish that the interplay of phagocytosis and autophagy within the RPE is required for both POS degradation and the maintenance of retinoid levels to support vision.

Age-dependent changes in FasL (CD95L) modulate macrophage function in a model of age-related macular degeneration

PURPOSE

We examined the effect of aging on Fas ligand (FasL) function in a mouse model of choroidal neovascularization (CNV).

METHODS

Young and aged mice were laser treated to induce CNV. Bone marrow chimeras were performed between young and aged mice. FasL protein expression was examined in the eye and soluble FasL (sFasL) was measured in the blood. Young and aged mice were treated with a matrix metalloprotease (MMP) inhibitor and systemic sFasL was neutralized by antibody treatment. Macrophages from young and aged mice were tested for sFasL-mediated cytokine production and migration.

RESULTS

The elevated CNV response observed with aging was dependent on bone marrow-derived cells. FasL expression in the eye was increased with age, but decreased following laser treatment. Aged mice had higher levels of sFasL in the blood compared to young mice. Systemic treatment with an MMP inhibitor decreased bloodborne sFasL, and reduced CNV in young and aged mice. Systemic neutralization of sFasL reduced CNV only in aged mice. sFasL increased cytokine production in aged macrophages and proangiogenic M2 macrophages. Aged M2 macrophages had elevated Fas (CD95) expression and displayed increased migration in response to sFasL compared to M1 macrophages derived from young animals.

CONCLUSIONS

Age modulates FasL function where increased MMP cleavage leads to a loss of function in the eye. The released form of FasL (sFasL) preferentially induces the migration of proangiogenic M2 macrophages into the laser lesions and increases proangiogenic cytokines promoting CNV. FasL may be a viable target for therapeutic intervention in aged-related neovascular disease.

The Rd8 mutation of the Crb1 gene is present in vendor lines of C57BL/6N mice and embryonic stem cells, and confounds ocular induced mutant phenotypes

PURPOSE

We noted an unexpected inheritance pattern of lesions in several strains of gene-manipulated mice with ocular phenotypes. The lesions, which appeared at various stages of backcross to C57BL/6, bore resemblance to the rd8 retinal degeneration phenotype. We set out to examine the prevalence of this mutation in induced mutant mouse lines, vendor C57BL/6 mice and in widely used embryonic stem cells.

METHODS

Ocular lesions were evaluated by fundus examination and histopathology. Detection of the rd8 mutation at the genetic level was performed by PCR with appropriate primers. Data were confirmed by DNA sequencing in selected cases.

RESULTS

Analysis of several induced mutant mouse lines with ocular disease phenotypes revealed that the disease was associated 100% with the presence of the rd8 mutation in the Crb1 gene rather than with the gene of interest. DNA analysis of C57BL/6 mice from common commercial vendors demonstrated the presence of the rd8 mutation in homozygous form in all C57BL/6N substrains, but not in the C57BL/6J substrain. A series of commercially available embryonic stem cells of C57BL/6N origin and C57BL/6N mouse lines used to generate ES cells also contained the rd8 mutation. Affected mice displayed ocular lesions typical of rd8, which were detectable by funduscopy and histopathology as early as 6 weeks of age.

CONCLUSIONS

These findings identify the presence of the rd8 mutation in the C57BL/6N mouse substrain used widely to produce transgenic and knockout mice. The results have grave implications for the vision research community who develop mouse lines to study eye disease, as presence of rd8 can produce significant disease phenotypes unrelated to the gene or genes of interest. It is suggested that researchers screen for rd8 if their mouse lines were generated on the C57BL/6N background, bear resemblance to the rd8 phenotype, or are of indeterminate origin.

Cell death in the maintenance and abrogation of tolerance: the five Ws of dying cells

The mammalian immune system continually faces death in the form of its own dead and dying cells that arise during normal tissue turnover, infections, cellular damage, and cancer. Complex decisions must then be made that will permit a protective response to pathogens, while at the same time destroying tumors but not attacking vital systems of the host that could lead to autoimmunity. By using an investigative technique termed the five Ws (who, what, when, where, and why), we will examine how the immune system responds to antigens generated via cell death. This analysis will give us a better understanding of the molecular differences fundamental to tolerogenic or immunogenic cell death, the cells that sense and react to the dead cells, and the consequences of these fundamental elements on the maintenance or abrogation of tolerance.

The plasticity of regulatory T cell function

Regulatory T cells (T(regs)) can suppress a wide variety of cell types, in diverse organ sites and inflammatory conditions. Whereas T(regs) possess multiple suppressive mechanisms, the number required for maximal function is unclear. Furthermore, whether any interrelationship or cross-regulatory mechanisms exist to orchestrate and control their utilization is unknown. In this study, we assessed the functional capacity of T(regs) lacking the ability to secrete both IL-10 and IL-35, which individually are required for maximal T(reg) activity. Surprisingly, IL-10/IL-35 double-deficient T(regs) were fully functional in vitro and in vivo. Loss of IL-10 and IL-35 was compensated for by a concurrent increase in cathepsin E (Ctse) expression, enhanced TRAIL (Tnfsf10) expression, and soluble TRAIL release, rendering IL-10/IL-35 double-deficient T(regs) functionally dependent on TRAIL in vitro and in vivo. Lastly, whereas C57BL/6 T(regs) are normally IL-10/IL-35 dependent, BALB/c T(regs), which express high levels of cathepsin E and enhanced TRAIL expression, are partially TRAIL dependent by default. These data reveal that cross-regulatory pathways exist that control the utilization of suppressive mechanisms, thereby providing T(reg) functional plasticity.

Armed response: how dying cells influence T-cell functions

Immune responses during infection, injury, and cancer proceed in the presence of tissue injury and cell death. Consequently, the system must deal with its own dead cells while it determines the appropriate response to the invader. As apoptotic cells are known to induce immune tolerance and necrotic cells can be potent stimulators of immunity, this decision becomes more complex. The key to understanding the immunologic choices made during cell death is to examine the mechanisms of tolerance induction by dying cells and then relate them to the mechanisms of immunity. Ideally, immunogenic cell death should be directed toward tumor cells and infected cells, whereas tolerogenic cell death should be associated with preventing unwanted immune responses to self. In this review, we discuss how the decision is made by focusing on the biochemical process of cell death and how its key components can influence both tolerance and immunity.

Systemic immunological tolerance to ocular antigens is mediated by TRAIL-expressing CD8+ T cells

Systemic immunological tolerance to Ag encountered in the eye restricts the formation of potentially damaging immune responses that would otherwise be initiated at other anatomical locations. We previously demonstrated that tolerance to Ag administered via the anterior chamber (AC) of the eye required Fas ligand-mediated apoptotic death of inflammatory cells that enter the eye in response to the antigenic challenge. Moreover, the systemic tolerance induced after AC injection of Ag was mediated by CD8(+) regulatory T cells. This study examined the mechanism by which these CD8(+) regulatory T cells mediate tolerance after AC injection of Ag. AC injection of Ag did not prime CD4(+) T cells and led to increased TRAIL expression by splenic CD8(+) T cells. Unlike wild-type mice, Trail(-/-) or Dr5(-/-) mice did not develop tolerance to Ag injected into the eye, even though responding lymphocytes underwent apoptosis in the AC of the eyes of these mice. CD8(+) T cells from Trail(-/-) mice that were first injected via the AC with Ag were unable to transfer tolerance to naive recipient wild-type mice, but CD8(+) T cells from AC-injected wild-type or Dr5(-/-) mice could transfer tolerance. Importantly, the transferred wild-type (Trail(+/+)) CD8(+) T cells were also able to decrease the number of infiltrating inflammatory cells into the eye; however, Trail(-/-) CD8(+) T cells were unable to limit the inflammatory cell ingress. Together, our data suggest that "helpless" CD8(+) regulatory T cells generated after AC injection of Ag enforce systemic tolerance in a TRAIL-dependent manner to inhibit inflammation in the eye.

TRAIL-expressing CD8+ T cells mediate tolerance following soluble peptide-induced peripheral T cell deletion

Peripheral tolerance controls the action of self-reactive T cells that escape thymic deletion. We showed previously that deletion of Ag-specific CD4+ T cells induced a CD8+ T(reg) population that maintained tolerance by deleting T cells with the same Ag specificity. The present study explored the mechanism of action of these CD8+ T(reg). Following OT-II T cell deletion by soluble OVA₃₂₃₋₃₃₉, B6 mice were unresponsive to challenge after CFA/OVA immunization, and Trail⁻/⁻ or Dr5⁻/⁻ mice were immune, although all strains displayed similar OT-II peripheral deletion. Interestingly, B6 mice remained tolerant to OVA even after a second infusion of OT-II T cells. Tolerance could be transferred to naïve recipients using CD8+ T cells from B6 or Dr5⁻/⁻ mice that experienced peptide-induced peripheral OT-II deletion but not from Trail⁻/⁻ mice. Subsequent investigation found that the mechanism of action of the CD8+ T(reg) was TRAIL-mediated OT-II T cell deletion in a TCR-specific manner. Furthermore, the tolerance was transient, as it was established by 14 days after peptide injection but lost by Day 56. Together, these data provide evidence to suggest that the mechanism behind transient peripheral tolerance induced following T cell deletion is the cytotoxic activity of TRAIL-expressing CD8+ T(reg).

Sepsis-induced apoptosis leads to active suppression of delayed-type hypersensitivity by CD8+ regulatory T cells through a TRAIL-dependent mechanism

Patients who survive severe sepsis often display severely compromised immune function. One hallmark of such immune suppression in septic patients is an impaired delayed-type hypersensitivity (DTH) response, manifested by a loss of skin testing to recall Ags. Because sepsis induces significant apoptosis in lymphoid and myeloid cells, and apoptotic cells are themselves tolerogenic, we tested the hypothesis that suppression of DTH is mediated by tolerogenic properties of the apoptotic cells generated during sepsis. Mice subjected to cecal ligation and puncture demonstrated a loss of DTH for the 7 d following cecal ligation and puncture; however, the immune response returned to normal by day 10. Blocking sepsis-induced apoptosis via Bcl-2 overexpression or Bim deficiency prevented the loss of DTH. Importantly, injection of apoptotic cells into Bim-/- mice prevented an effective DTH response, thereby suggesting a causal link between apoptotic cells and immune suppression. Surprisingly, when TRAIL null mice were examined, we found that these animals had significant apoptosis but retained their DTH responses. Further studies revealed that apoptotic cells generated during sepsis induced a CD8+ regulatory T cell that suppressed DTH by TRAIL production. These results establish a link between apoptotic cells and immune suppression during sepsis and suggest TRAIL may be a viable therapeutic target for boosting the adaptive immune response following sepsis.

IL-7 promotes T cell viability, trafficking, and functionality and improves survival in sepsis

Sepsis is a highly lethal disorder characterized by widespread apoptosis-induced depletion of immune cells and the development of a profound immunosuppressive state. IL-7 is a potent antiapoptotic cytokine that enhances immune effector cell function and is essential for lymphocyte survival. In this study, recombinant human IL-7 (rhIL-7) efficacy and potential mechanisms of action were tested in a murine peritonitis model. Studies at two independent laboratories showed that rhIL-7 markedly improved host survival, blocked apoptosis of CD4 and CD8 T cells, restored IFN-gamma production, and improved immune effector cell recruitment to the infected site. Importantly, rhIL-7 also prevented a hallmark of sepsis (i.e., the loss of delayed-type hypersensitivity), which is an IFN-gamma- and T cell-dependent response. Mechanistically, rhIL-7 significantly increased the expression of the leukocyte adhesion markers LFA-1 and VLA-4, consistent with its ability to improve leukocyte function and trafficking to the infectious focus. rhIL-7 also increased the expression of CD8. The potent antiapoptotic effect of rhIL-7 was due to increased Bcl-2, as well as to a dramatic decrease in sepsis-induced PUMA, a heretofore unreported effect of IL-7. If additional animal studies support its efficacy in sepsis and if current clinical trials continue to confirm its safety in diverse settings, rhIL-7 should be strongly considered for clinical trials in sepsis.

Fractures of the acetabulum in patients aged 60 years and older: an epidemiological and radiological study

Using a prospective database of 1309 displaced acetabular fractures gathered between 1980 and 2007, we calculated the annual mean age and annual incidence of elderly patients > 60 years of age presenting with these injuries. We compared the clinical details and patterns of fracture between patients > 60 years of age (study group) with those < 60 years (control group). We performed a detailed evaluation of the radiographs of the older group to determine the incidence of radiological characteristics which have been previously described as being associated with a poor patient outcome. In all, 235 patients were > 60 years of age and the remaining 1074 were < 60 years. The incidence of elderly patients with acetabular fractures increased by 2.4-fold between the first half of the study period and the second half (10% (62) vs 24% (174), p < 0.001). Fractures characterised by displacement of the anterior column were significantly more common in the elderly compared with the younger patients (64% (150) vs 43% (462), respectively, p < 0.001). Common radiological features of the fractures in the study group included a separate quadrilateral-plate component (50.8% (58)) and roof impaction (40% (46)) in the anterior fractures, and comminution (44% (30)) and marginal impaction (38% (26)) in posterior-wall fractures. The proportion of elderly patients presenting with acetabular fractures increased during the 27-year period. The older patients had a different distribution of fracture pattern than the younger patients, and often had radiological features which have been shown in other studies to be predictive of a poor outcome.

Targeting immune privilege to prevent pathogenic neovascularization

PURPOSE. Current studies suggest that the immune system plays a critical role in blinding eye disorders. The eye is an immune-privileged site, and FasL expression is a major part of that mechanism because Fas/FasL interactions regulate inflammation and neovascularization, preventing damage to delicate ocular structures. These studies were undertaken to test the idea that modulating immune privilege might be an effective therapeutic approach to pathogenic angiogenesis in the eye. METHODS. C57BL/6 mice or FasL-defective B6-gld mice were laser treated to induce choroidal neovascularization (CNV). Mice were injected with cytotoxic FasL in the vitreous cavity or were treated with oral doxycycline in the drinking water. They were evaluated for CNV 7 days later. In some experiments eye tissue was harvested and evaluated for FasL expression, macrophage influx by immunohistochemistry, and release of sFasL. RESULTS. Injection of cytotoxic FasL successfully prevented neovascularization in a mouse model of CNV. Oral doxycycline increased functional FasL in the eye and substantially inhibited neovascularization. Doxycycline treatment increased FasL expression on the RPE cells and reduced circulating and tissue-associated sFasL. Treatment was ineffective in B6-gld mice, demonstrating that CNV inhibition was mediated by FasL. CONCLUSIONS. Targeting immune privilege using cytotoxic molecules or by increasing expression of the proapoptotic protein FasL may be a viable approach to treating neovascular eye disease.

IL-15 prevents apoptosis, reverses innate and adaptive immune dysfunction, and improves survival in sepsis

IL-15 is a pluripotent antiapoptotic cytokine that signals to cells of both the innate and adaptive immune system and is regarded as a highly promising immunomodulatory agent in cancer therapy. Sepsis is a lethal condition in which apoptosis-induced depletion of immune cells and subsequent immunosuppression are thought to contribute to morbidity and mortality. This study tested the ability of IL-15 to block apoptosis, prevent immunosuppression, and improve survival in sepsis. Mice were made septic using cecal ligation and puncture or Pseudomonas aeruginosa pneumonia. The experiments comprised a 2 x 2 full factorial design with surgical sepsis versus sham and IL-15 versus vehicle. In addition to survival studies, splenic cellularity, canonical markers of activation and proliferation, intracellular pro- and antiapoptotic Bcl-2 family protein expression, and markers of immune cell apoptosis were evaluated by flow cytometry. Cytokine production was examined both in plasma of treated mice and splenocytes that were stimulated ex vivo. IL-15 blocked sepsis-induced apoptosis of NK cells, dendritic cells, and CD8 T cells. IL-15 also decreased sepsis-induced gut epithelial apoptosis. IL-15 therapy increased the abundance of antiapoptotic Bcl-2 while decreasing proapoptotic Bim and PUMA. IL-15 increased both circulating IFN-gamma, as well as the percentage of NK cells that produced IFN-gamma. Finally, IL-15 increased survival in both cecal ligation and puncture and P. aeruginosa pneumonia. In conclusion, IL-15 prevents two immunopathologic hallmarks of sepsis, namely, apoptosis and immunosuppression, and improves survival in two different models of sepsis. IL-15 represents a potentially novel therapy of this highly lethal disorder.

Activation-induced CD154 expression abrogates tolerance induced by apoptotic cells

The decision to generate a productive immune response or tolerance often depends on the context in which T cells first see Ag. Using a classical system of tolerance induction, we examined the immunological consequence of Ag encountered in the presence of naive or activated apoptotic cells. Naive apoptotic cells induced tolerance when injected i.v.; however, previously activated apoptotic cells induced immunity. Further analysis revealed a key role for CD154, as tolerance resulted after i.v. injection of either naive or activated apoptotic CD154(-/-) T cells, while coinjection of an agonistic anti-CD40 mAb with naive apoptotic T cells induced robust immunity. Dendritic cells fed activated apoptotic T cells in vitro produced IL-12p40 in a CD154-dependent manner, and the use of IL-12p40(-/-) mice or mAb-mediated neutralization of IL-12 revealed a link between CD154, IL-12, and the ability of activated apoptotic T cells to induce immunity rather than tolerance. Collectively, these results show that CD154 expression on apoptotic T cells can determine the outcome of an immune response to Ag recognized within the context of the apoptotic cells and suggest that the balance between naive and activated apoptotic T cells may dictate whether a productive immune response is encouraged.

Identification of an IFN-gamma-producing neutrophil early in the response to Listeria monocytogenes

IFN-gamma plays a critical role during the immune response to infection with Listeria monocytogenes. Early in the innate response NK cells are thought to be a primary source of IFN-gamma; however, protection can be mediated by the presence of significant numbers of primed IFN-gamma-secreting CD8(+) T cells. In this report, we examined the early response to Listeria and found that 18 h after infection spleens contain CD11b(+), Gr-1(high), or Ly6G(+) cells that produce significant IFN-gamma. Morphological analysis of sorted Gr-1(high)IFN-gamma(+) and Gr-1(low)IFN-gamma(+) or Ly6G(+)IFN-gamma(+) cells confirmed that these cells were neutrophils. The importance of IFN-gamma production by these cells was further tested using adoptive transfer studies. Transfer of purified neutrophils from Ifng(+/+) mice led to increased bacterial clearance in Ifng(-/-) mice. Transfer of Ifng(-/-) neutrophils provided no such protection. We conclude that neutrophils are an early source of IFN-gamma during Listeria infection and are important in providing immune protection.

Sepsis-induced human lymphocyte apoptosis and cytokine production in "humanized" mice

Sepsis is the leading cause of death in critically ill patients in the United States with over 210,000 deaths annually. One stumbling block to an effective therapy of sepsis has been the lack of a clinically relevant animal model. There are important distinctions in the mouse versus human immune system regarding the host response to invading pathogens. These differences may explain the disappointing results in many sepsis clinical trials despite the clear efficacy of these agents in mouse models of sepsis. The purpose of the present study was to develop a "humanized" mouse model of sepsis and to determine if the model recapitulated the major findings of lymphocyte apoptosis and cytokine response that exist in patients with sepsis. Two-day-old NOD-scid IL2rgamma(null) mice received an adoptive transfer of hCD34(+) hematopoietic cord blood stem cells. These mice acquired a functional human innate and adaptive immune system, as evidenced by the development of all lineages of human immune cells as well as by mounting a DTH response. Eight weeks post-transfer, mice were made septic using the highly clinical relevant CLP model of sepsis, and sepsis induced marked elevations in human pro- and anti-inflammatory cytokines as well as a dramatic increase in human T and B cell apoptosis. Collectively, these results show that the humanized mouse model recapitulates many of the classic findings in patients with sepsis. Therefore, it represents an advanced, clinically relevant model for mechanistic studies of sepsis and testing of novel therapies.

TRAIL-expressing CD8+ regulatory T cells participate in the maintenance of ocular immune privilege (39.40)

Immunological tolerance to antigens encountered in the eye, the prototypic immune privileged site, inhibits potentially damaging immune responses that would otherwise be initiated to these antigens at other anatomical locations. The induction of tolerance depends on the FasL-mediated apoptotic death of inflammatory cells that enter the eye in response to antigenic challenge. Moreover, the tolerance is not due to the failure to prime, but is mediated by the generation of CD8+ regulatory T cells (Treg) that can transfer tolerance to a naïve recipient. The present study investigated the mechanism of action of the induced CD8+ Treg. Unlike wildtype (WT) mice, TRAIL-/- or DR5-/- mice did not develop tolerance to antigen injected into the eye, even though the responding lymphocytes underwent apoptosis in the eyes of all mice. We also found that splenocytes from tolerant WT mice produced significantly less IFN, compared to TRAIL-/- mice, after restimulation. Interestingly, CD8+ T cells from TRAIL-/- mice were unable to transfer tolerance to naïve recipient WT mice, but CD8+ T cells from WT or DR5-/- mice could transfer tolerance. Subsequent investigation found that tolerant animals contained CD8+ Treg cells capable of killing antigen-pulsed splenocytes via TRAIL. Together, our data indicate that CD8+ Treg enforce tolerance in a TRAIL-dependent manner to inhibit inflammation in the eye.

Peptide-induced peripheral T cell deletion leads to tolerance mediated by TRAIL-expressing CD8+ regulatory T cells (89.16)

Peripheral tolerance controls self-reactive T cells that escape thymic deletion. We previously showed that deletion of Ag-specific CD4+ T cells after soluble Ag injection leads to immune tolerance mediated by a CD8+ Treg population in response to the processing and presentation of the apoptotic T cells by the immune system. The CD8+ Treg maintain long-term tolerance to subsequent Ag challenge by deleting T cells that share the same Ag specificity. The present study explored the mechanism of action of these CD8+ Treg. Our data show OT-II T cells expand and contract equally in wildtype (WT) and TRAIL-/- mice after OVA323-339 injection. After contraction, WT mice were tolerant to subsequent OVA/CFA immunization, even when given fresh OT-II T cells, but TRAIL-/- mice displayed immunity. Analysis of the tolerant B6 mice found the transferred OT-II T cells did not undergo secondary expansion after OVA/CFA immunization, but secondary T cell expansion was evident in the non-tolerant TRAIL-/- mice. Subsequent investigation found direct killing of freshly transferred OT-II T cells in a TCR-specific manner in tolerant WT mice, but not in TRAIL-/- mice, and only CD8+ T cells from tolerant WT mice could transfer tolerance to naïve recipients. These data provide evidence for a novel mechanism of peripheral tolerance maintained through the cytotoxic activity of TRAIL-expressing CD8+ Treg against T cells of the same Ag specificity.

Differential lymphopenia-induced homeostatic proliferation for CD4+ and CD8+ T cells following septic injury

Sepsis is a severe, life-threatening infection and a leading cause of death in hospitals. A hallmark of sepsis is the profound apoptosis-induced depletion of lymphocytes generating a lymphopenic environment. As lymphopenia can induce nonantigen-driven homeostatic proliferation (HP), we examined this process during sepsis. CD4(+) and CD8(+) T cells, which were depleted within 24 h of sepsis induction, remained at significantly reduced levels until Day 21 when normal numbers were detected. When HP was examined, naïve CD8(+) T cells proliferated between Day 7 and Day 21 post-cecal ligation and puncture, developing into memory cells with relatively few cells expressing an activation phenotype. Conversely, naïve CD4(+) T cells did not undergo HP, but proportionally higher numbers expressed activation markers. Adoptive transfer studies revealed that T cells from mice that had recovered from sepsis were not protective when transferred to naïve mice undergoing sepsis. In addition, the TCR repertoire was not skewed toward any specific Vbeta type but resembled the repertoire found in normal mice, suggesting that T cells were not primed to antigens resulting from the infection. Interestingly, depletion of endogenous CD8(+) but not CD4(+) T cells restored the ability of naive CD4(+) T cells to undergo HP, increasing the number of CD4(+) T cells with memory but not activation markers. We conclude that homeostatic control in the postseptic environment permits recovery of the T cell repertoire to normal levels without generating antigen-specific memory or aberrant T cell specificities. Restoration of homeostatic control mechanisms might be a rational therapy for this disorder.

Induction of immunological tolerance by apoptotic cells requires caspase-dependent oxidation of high-mobility group box-1 protein

The mammalian immune system discriminates between modes of cell death; necrosis often results in inflammation and adaptive immunity, whereas apoptosis tends to be anti-inflammatory and promote immune tolerance. We have examined apoptosis for the features responsible for tolerance; specifically, we looked at the roles of caspases and mitochondria. Our results show that caspase activation targeted the mitochondria to produce reactive oxygen species (ROS), which were critical to tolerance induction by apoptotic cells. ROS oxidized the potential danger signal high-mobility group box-1 protein (HMGB1) released from dying cells and thereby neutralized its stimulatory activity. Apoptotic cells failed to induce tolerance and instead stimulated immune responses by scavenging or by mutating a mitochondrial caspase target protein when ROS activity was prohibited. Similarly, blocking sites of oxidation in HMGB1 prevented tolerance induction by apoptotic cells. These results suggest that caspase-orchestrated mitochondrial events determine the impact of apoptotic cells on the immune response.

Senescence regulates macrophage activation and angiogenic fate at sites of tissue injury in mice

Abnormal angiogenesis plays a key role in diseases of aging such as heart disease, certain cancers, and eye diseases including age-related macular degeneration. Macrophages have been shown previously to be both anti- and proangiogenic, and their role in regulating angiogenesis at sites of tissue injury is critical and complex. In this study, we analyzed cytokine gene expression patterns of mouse macrophages by real-time quantitative PCR and tested the functional effects of senescence on gene expression and macrophage polarization. Following laser injury to the retina, IL-10 was upregulated and Fas ligand (FasL), IL-12, and TNF-alpha were downregulated in ocular macrophages of old mice (>18 months of age). Downregulation of FasL on macrophages led to a loss of the antiangiogenic phenotype, as evidenced by the inability of these macrophages to inhibit vascular endothelial cells. Our results demonstrate that senescence, FasL, and IL-10 are key determinants of macrophage function that alter the growth of abnormal postdevelopmental blood vessels in disease processes including blinding eye disease.

Apoptotic Cells Induce Tolerance by Generating Helpless CD8+ T Cells That Produce TRAIL

The decision to generate a productive immune response or immune tolerance following pathogenic insult often depends on the context in which T cells first encounter Ag. The presence of apoptotic cells favors the induction of tolerance, whereas immune responses generated with necrotic cells promote immunity. We have examined the tolerance induced by injection of apoptotic cells, a system in which cross-presentation of Ag associated with the dead cells induces CD8+ regulatory (or suppressor) T cells. We observed that haptenated apoptotic cells induced CD8+ suppressor T cells without priming CD4+ T cells for immunity. These CD8+ T cells transferred unresponsiveness to naive recipients. In contrast, haptenated necrotic cells stimulated immunity, but induced CD8+ suppressor T cells when CD4+ T cells were absent. We further found that CD8+ T cells induced by these treatments displayed a "helpless CTL" phenotype and suppress the immune response by producing TRAIL. Animals deficient in TRAIL were resistant to tolerance induction by apoptotic cells. Thus, the outcome of an immune response taking place in the presence of cell death can be determined by the presence of CD4+-mediated Th cell function.

The role of TCR engagement and activation-induced cell death in sepsis-induced T cell apoptosis

Sepsis induces extensive apoptosis in T and B cells suggesting that the loss of immune effector cells could be one explanation for the profound immunosuppression observed in this disorder. Unfortunately, the mechanisms responsible for lymphocyte apoptosis in sepsis remain unknown. In T cells, apoptosis can occur through activation-induced cell death (AICD) in which engagement of the Ag receptors by cognate Ag or polyclonal activators such as bacteria-derived superantigens induces activation, proliferation, and apoptosis. We examined whether proliferation and AICD are necessary for apoptotic cell death in sepsis using normal and TCR transgenic mice. Results show that although sepsis resulted in activation of a small percentage of T cells, no proliferation was detected during the first 48 h following onset, a time when extensive apoptosis is observed. We also observed that T cells do not enter the cell cycle, and stimulation via the TCR in TCR transgenic animals does not enhance or decrease cell death in sepsis. Interestingly, T cells recovered from septic mice retained their ability to proliferate and synthesize cytokines albeit at reduced levels. With the exception of IL-10, which was increased in lymphocytes from mice with sepsis, sepsis caused a decrease in the production of both proinflammatory and anti-inflammatory cytokines. We conclude that lymphocyte apoptosis in sepsis does not require proliferation, TCR engagement, or AICD. Thus the immunosuppression observed in sepsis cannot be the result of T cell deletion via the TCR.

The role of Fas ligand and TNF-related apoptosis-inducing ligand (TRAIL) in the ocular immune response

The host response to pathogenic insults involves complex inflammatory responses and cellular immune reactions. While these are central to host defense and vital to clearing dangerous invaders, they are often associated with nonspecific injury to nearby tissue. These localized reactions function to successfully deal with pathogens before they spread to other areas. They are generally effective since most organ systems can tolerate these responses without permanent consequences. There are sites, however, that prohibit the spread of inflammation because these episodes can threaten organ integrity and function. The most prominent examples of these are the eye, brain, and reproductive organs (testis and ovary) where even minor bouts of inflammation can have long-term consequences on the survival of the organism. In these areas, immune responses either do not proceed, or proceed in a manner different from other areas; thus, they are called 'immunologically privileged'. Studies by a number of laboratories have determined that there are a number of mediators of ocular immune privilege. These include locally produced immunosuppressive cytokines, neuropeptides, limited expression of major histocompatibility complex class I and class II, complementregulatory proteins, immune deviation, natural killer cell inhibitors, and the expression of the death-inducing ligands Fas ligand (FasL) and tumor necrosis factor (TNF)-related apoptosisinducing ligand (TRAIL). The death-inducing molecules are poised to effectively deal with inflammatory cells once they pass the natural barriers of the eye, and effectively limit the spread of inflammatory cells and tumor cells within the confines of the eye by inducing apoptosis. The function of FasL and TRAIL will be the subject of this chapter.

A vision of cell death: Fas ligand and immune privilege 10 years later

Immune privilege is a term applied to organs that have a unique relationship with the immune response. These sites prohibit the spread of inflammation, since even minor episodes can threaten organ integrity and function. Once thought to be a passive process relying on physical barriers, immune privilege is now viewed as an active process, which uses multiple mechanisms to maintain organ function. The prototypic organ of immune privilege has been the eye, where the spread of inflammation can threaten vision. Nearly 10 years ago, we discussed the finding that Fas ligand (FasL) was constitutively expressed in the eye and played a major role in immune privilege by inducing apoptosis in inflammatory cells that enter the eye. In this review, we reexamine the original evidence for the role of FasL in immune privilege, update progress on some of the concepts, and discuss some of the issues that remain unresolved.

Signals from dying cells: tolerance induction by the dendritic cell

Uncontrolled immune responses are central to the pathogenesis of sustained viral infection, tumor growth, autoimmune diseases, and organ rejection. Dendritic cells (DCs) are critical to determining the outcome of immune responses by directing the function of T-cells. When DCs encounter antigen together with maturation signals immunity results; however, when they encounter apoptotic cells immune tolerance can be induced. It is thought that this is a mechanism to help prevent responses to self antigens because apoptotic cells can enter antigen-processing pathways. Our interest is the mechanisms by which DCs induce tolerance to the antigens associated with apoptotic cells. Here, we examine evidence showing a pivotal role for interleukin-10 (IL-10) in regulating the response of DCs to apoptotic cells. We show that although tolerogenic DCs are CD11c+ and CD8alpha+, the important DCs reside in a subpopulation of these cells that also express IL-10R. Current studies are designed to isolate and characterize this extremely small, but potent, subpopulation of tolerance-inducing DCs.

Agonistic monoclonal antibody against CD40 receptor decreases lymphocyte apoptosis and improves survival in sepsis

Sepsis causes a marked apoptosis-induced depletion of lymphocytes. The degree of lymphocyte apoptosis during sepsis strongly correlates with survival. CD40, a member of the TNFR family, is expressed on APCs and has potent antiapoptotic activity. In this study we determined whether an agonistic Ab against CD40 could protect lymphocytes from sepsis-induced apoptosis. Secondly, we examined potential antiapoptotic mechanisms of the putative protection. Lastly, we aimed to determine whether anti-CD40 treatment could improve survival in sepsis. CD1 mice were made septic by the cecal ligation and puncture method and treated postoperatively with anti-CD40 Ab. Treatment with anti-CD40 completely abrogated sepsis-induced splenic B cell death and, surprisingly, decreased splenic and thymic T cell death as well (p < 0.001). To investigate the mechanism of protection of anti-CD40 therapy on T cells, CD40 receptor expression was examined. As anticipated, the CD40 receptor was constitutively expressed on B cells, but, unexpectedly, splenic and thymic T cells were found to express CD40 receptor during sepsis. Furthermore, CD4+CD8- T cells were the predominant subtype of T cells expressing CD40 receptor during sepsis. Additionally, the antiapoptotic protein Bcl-x(L) was found to be markedly increased in splenic B and T cells as well as in thymic T cells after treatment with anti-CD40 Ab (p < 0.0025). Lastly, mice that were made septic in a double injury model of sepsis had improved survival after treatment with anti-CD40 as compared with controls (p = 0.05). In conclusion, anti-CD40 treatment increases Bcl-x(L), provides nearly complete protection against sepsis-induced lymphocyte apoptosis, and improves survival in sepsis.

Macrophages inhibit neovascularization in a murine model of age-related macular degeneration

BACKGROUND

Age-related macular degeneration (AMD) is the leading cause of blindness in people over 50 y of age in at least three continents. Choroidal neovascularization (CNV) is the process by which abnormal blood vessels develop underneath the retina. CNV develops in 10% of patients with AMD but accounts for up to 90% of the blindness from AMD. Although the precise etiology of CNV in AMD remains unknown, the macrophage component of the inflammatory response, which has been shown to promote tumor growth and support atherosclerotic plaque formation, is thought to stimulate aberrant angiogenesis in blinding eye diseases. The current theory is that macrophage infiltration promotes the development of neovascularization in CNV.

METHODS AND FINDINGS

We examined the role of macrophages in a mouse model of CNV. IL-10(-/-) mice, which have increased inflammation in response to diverse stimuli, have significantly reduced CNV with increased macrophage infiltrates compared to wild type. Prevention of macrophage entry into the eye promoted neovascularization while direct injection of macrophages significantly inhibited CNV. Inhibition by macrophages was mediated by the TNF family death molecule Fas ligand (CD95-ligand).

CONCLUSIONS

Immune vascular interactions can be highly complex. Normal macrophage function is critical in controlling pathologic neovascularization in the eye. IL-10 regulates macrophage activity in the eye and is an attractive therapeutic target in order to suppress or inhibit CNV in AMD that can otherwise lead to blindness.

Abnormal anterior chamber associated immune deviation (ACAID) in 129-strain mice

PURPOSE

To characterize anterior chamber immune deviation (ACAID) in 129-strain and mixed 129-strain mice.

METHODS

ACAID was assayed using standard protocols with herpes simplex-1 (HSV-1) and trinitrophenol-hapten-spleen cells (TNP-spleen) in C57B1/6, 129P2, 129X1, and intercrossed strains. Systemic tolerance induction was assayed using an ultraviolet light skin tolerance protocol to 2,-4,6-trinitro-l-chlorobenzene (TNCB).

RESULTS

129X1 and C57Bl/6xl29Xl Fl mice did not show ACAID to HSV-1. C57Bl/6xl29P2 mice did not show ACAID to TNP-spleen. C57Bl/6xl29P2 mice did show normal peripheral immune deviation to TNCB. (C57Bl/6xl29Xl) x C57B1/6 N2 backcrossed mice showed a bimodal ACAID response to HSV-1 suggesting a single dominant allele in the 129X1 background responsible for suppressing ACAID.

CONCLUSION

ACAID to multiple antigens is significantly reduced in 129-strain mice and their outcrossed progeny. Since 129-strain embryonic stem cells are widely used to generate knockout and transgenic mice, care must be taken to extensively backcross resultant strains in order to assess the effect of particular genes on ACAID.

The role of Fas ligand as an effector molecule in corneal graft rejection

Previous studies have shown that the expression of Fas ligand (FasL; CD95L) by donor corneas is critical to their survival when placed on allogeneic recipients. Since there have been reports that the cornea expresses Fas, we tested the idea that FasL on lymphoid cells could be an effector molecule during rejection episodes. When FasL defective BALB/c-gld mice were engrafted with allogeneic corneas, significantly more of these corneas were accepted than by normal BALB/c mice. However, this was not due to impaired FasL-mediated effector function in these mice as the allogeneic corneas did not express detectable Fas by Western blot or RT-PCR analysis. Furthermore, donor corneas without Fas were given no survival advantage, but were rejected similar to wild-type donor allogeneic corneas. Examination of the T cell compartment in gld mice revealed that these cells express higher levels of Fas and are more susceptible to Fas-mediated death than wild-type cells. These results indicate that FasL is not an effector molecule in corneal graft rejection and that gld mice show reduced graft rejection due to greater susceptibility of their T cells to Fas-mediated apoptosis.

Peripheral deletion of antigen-specific T cells leads to long-term tolerance mediated by CD8+ cytotoxic cells

Peripheral deletion is one mechanism by which potentially self-reactive clones are removed whether they escape thymic deletion. We have examined the consequences of deleting Ag-specific T cells by i.v. injection of soluble Ag. Deletion of DO11.10 T cells by peptide was mediated predominately via a Fas/FasL mechanism. Animals that underwent deletion were tolerant to subsequent immunization with Ag, even when tolerant mice were given fresh Ag-specific DO11.10 T cells before immunization. Tolerance was mediated by CD8(+) T cells that killed the DO11.10-transgenic T cells in vivo. These data demonstrate that the programmed cell death of large numbers of T cells leads to peripheral tolerance mediated by CD8(+) CTLs.

Stimulation of neovascularization by the anti-angiogenic factor PEDF

PURPOSE

Examine the effect of (pigment epithelium-derived growth factor; PEDF) on laser-induced choroidal neovascularization (CNV).

METHODS

Adult C57Bl/6 mice were anesthetized and four laser spots were placed in each quadrant of the fundus with a krypton red laser (614 nm, 50 microm, 0.05 second, 200 mW). Animals were treated with various doses of PEDF administered with miniosmotic pumps implanted subcutaneously. Seven days after laser treatment, mice were perfused with 3% FITC high-molecular-weight dextran, the eyes enucleated, and neovascularization analyzed by confocal microscopy. Data were recorded as the volume of the neovascular complex. The effect of PEDF on endothelial cell migration, vascular tube formation in synthetic basement membrane, and VEGF production was also determined.

RESULTS

Mice receiving a lower dose of PEDF (90 microg/mL) had significantly decreased areas of CNV. A high dose of PEDF (360 microg/mL) significantly increased CNV, whereas an intermediate dose (180 microg/mL) of PEDF had no effect. PEDF inhibited endothelial cell migration and vascular tube formation at lower doses (0.5-5 microg/mL). High doses of PEDF (25-50 microg/mL) stimulated endothelial cell migration, enhanced vascular tube formation in vitro, and stimulated VEGF production from endothelial cells. Neutralizing anti-VEGF antibody completely reversed the stimulatory effects of high doses of PEDF on CNV in vivo.

CONCLUSIONS

PEDF demonstrates opposing effects on CNV and endothelial cell function. Whereas low doses are inhibitory, high doses can augment the development of the neovasculature. These results suggest that the effects of PEDF on neovascularization are more complex than originally believed and that caution should be exercised when PEDF therapies are considered.

Termination of Antigen-Specific Immunity by CD95 Ligand (Fas Ligand) and IL-10

Following elimination of a foreign invader, the immune system must return to its normal quiescent levels. This process requires removal of reactive immune cells when they are no longer needed. We have explored the role of Fas/Fas ligand (FasL) in terminating immunity and demonstrate that mice defective in these proteins have prolonged immune responses. Studies demonstrate that termination of immunity occurs via the interaction of Fas(+) lymphoid cells with FasL(+) nonlymphoid cells at the site of Ag challenge. Our results also show that FasL is absent in quiescent tissue but is rapidly up-regulated during the local immune reaction. This occurs through the production of IL-10. Thus, FasL and IL-10 work in concert to eliminate inflammatory cells and control the duration of an immune response.