Sialic Acid-Dependent Inhibition of T Cells by Exosomal Ganglioside GD3 in Ovarian Tumor Microenvironments.

The tumor microenvironment is rendered immunosuppressive by a variety of cellular and acellular factors that represent potential cancer therapeutic targets. Although exosomes isolated from ovarian tumor ascites fluids have been previously reported to induce a rapid and reversible T cell arrest, the factors present on or within exosomes that contribute to immunosuppression have not been fully defined. In this study, we establish that GD3, a ganglioside expressed on the surface of exosomes isolated from human ovarian tumor ascites fluids, is causally linked to the functional arrest of T cells activated through their TCR. This arrest is inhibited by Ab blockade of exosomal GD3 or by the removal of GD3+ exosomes. Empty liposomes expressing GD3 on the surface also inhibit the activation of T cells, establishing that GD3 contributes to the functional arrest of T cells independent of factors present in exosomes. Finally, we demonstrate that the GD3-mediated arrest of the TCR activation is dependent upon sialic acid groups, because their enzymatic removal from exosomes or liposomes results in a loss of inhibitory capacity. Collectively, these data define GD3 as a potential immunotherapeutic target.

Pregnancy-associated breast cancers are driven by differences in adipose stromal cells present during lactation.

INTRODUCTION: The prognosis of breast cancer is strongly influenced by the developmental stage of the breast when the tumor is diagnosed. Pregnancy-associated breast cancers (PABCs), cancers diagnosed during pregnancy, lactation, or in the first postpartum year, are typically found at an advanced stage, are more aggressive and have a poorer prognosis. Although the systemic and microenvironmental changes that occur during post-partum involution have been best recognized for their role in the pathogenesis of PABCs, epidemiological data indicate that PABCs diagnosed during lactation have an overall poorer prognosis than those diagnosed during involution. Thus, the physiologic and/or biological events during lactation may have a significant and unrecognized role in the pathobiology of PABCs. METHODS: Syngeneic in vivo mouse models of PABC were used to examine the effects of system and stromal factors during pregnancy, lactation and involution on mammary tumorigenesis. Mammary adipose stromal cell (ASC) populations were isolated from mammary glands and examined by using a combination of in vitro and in vivo functional assays, gene expression analysis, and molecular and cellular assays. Specific findings were further investigated by immunohistochemistry in mammary glands of mice as well as in functional studies using ASCs from lactating mammary glands. Additional findings were further investigated using human clinical samples, human stromal cells and using in vivo xenograft assays. RESULTS: ASCs present during lactation (ASC-Ls), but not during other mammary developmental stages, promote the growth of carcinoma cells and angiogenesis. ASCs-Ls are distinguished by their elevated expression of cellular retinoic acid binding protein-1 (crabp1), which regulates their ability to retain lipid. Human breast carcinoma-associated fibroblasts (CAFs) exhibit traits of ASC-Ls and express crabp1. Inhibition of crabp1in CAFs or in ASC-Ls abolished their tumor-promoting activity and also restored their ability to accumulate lipid. CONCLUSIONS: These findings imply that (1) PABC is a complex disease, which likely has different etiologies when diagnosed during different stages of pregnancy; (2) both systemic and local factors are important for the pathobiology of PABCs; and (3) the stromal changes during lactation play a distinct and important role in the etiology and pathogenesis of PABCs that differ from those during post-lactational involution.

Neovascular Glaucoma: A Rare Presenting Feature of Vogt-Koyanagi-Harada Syndrome.

Vogt-Koyanagi-Harada syndrome (VKH) is an uncommon multi-system autoimmune inflammatory disorder characterized by bilateral granulomatous panuveitis with serous retinal detachment accompanied by neurological, auditory, and cutaneous manifestations like headache, hearing loss, vitiligo, and poliosis. It has a female preponderance, usually in middle age. We report the case of a 20-year-old male who presented to us with rapidly progressive visual loss accompanying granular panuveitis, complicated cataract, and a mixed mechanism neovascular glaucoma with acute angle closure. He was treated for IOP control and underwent aggressive immunosuppression and, later, bilateral laser iridotomies. It wasn't until one month after the initial presentation that he presented with vitiligo and poliosis of the eyebrows and eyelashes, clinching the diagnosis of VKH syndrome. This case highlights the diagnostic challenge faced due to acute neovascular glaucoma being the initial presenting feature of VKH; hitherto not documented before, although acute angle closure glaucoma or crisis has occasionally been reported at presentation; the classical VKH presentation being an acute posterior segment uveitis or less commonly, a chronic, recurrent panuveitis presenting with/ without complications. This case underlines the importance of considering VKH syndrome in a patient with bilateral granulomatous panuveitis, as dermatological involvement can emerge later in the disease course, by which time vision might have already been compromised significantly.

Occlusio Pupillae: A Duet of Darkness Where the Patient Sees Naught, and the Doctor Discerns Not.

A 71-year-old, one-eyed female patient presented with a loss of vision in the right eye due to trauma 20 years ago and a progressive diminution of vision in the left eye over the past six years. An ambiguous history of some surgery performed on the left eye was elicited, with no available records, adding an element of uncertainty to this case. Visual acuity (VA) was noted as no light perception (No PL) in the right eye and light perception with accurate projection of rays (PL+, PR accurate) in the left eye. Anterior segment slit-lamp evaluation of the right eye showed a shrunken globe with low intraocular pressure (IOP). The left eye exhibited signs of chronic uveitis with occlusio pupillae, non-visualization of the lens, and a doubtful conjunctival bleb with scleral thinning superior to the limbus. B-scan evaluation was suggestive of phthisis in the right eye and an equivocal lens shadow in the left eye. A yttrium aluminum garnet (YAG) pupillary membranotomy was planned for the left eye under steroid cover and was cautiously attempted, successfully detaching the occlusio membrane and revealing an underlying complicated cataract beneath it. Post-laser, medical management included topical anti-glaucoma and steroid medications, along with systemic steroids. The VA improved from PL+, PR accurate to 3/60 (improving to 6/60 with a Retinal Acuity Meter). After stabilization of the uveitis over the next few weeks and under a steroid cover, a temporal clear-corneal phacoemulsification was cautiously performed with intra-operative management of the small pupil, and a hydrophobic lens was implanted. At one month post-surgery, the patient's best-corrected visual acuity had improved to 6/12 for distance and N6 for near. This report highlights a compelling instance wherein the neodymium:Yttrium-aluminum-garnet (Nd:YAG) laser was efficaciously employed for a lesser-known application in resolving a diagnostic dilemma and for instituting an interim treatment strategy in a challenging case involving a one-eyed patient prior to planning a definitive surgery. This case emphasizes the importance of thinking out of the box, ensuring comprehensive preoperative and careful intra-operative precautions in the management of patients diagnosed with complex ocular inflammatory conditions, so as to optimize visual outcomes, eventually resulting in achieving a gratifying reduction of visual disability and improvement of quality of life.

alpha3beta1 integrin-controlled Smad7 regulates reepithelialization during wound healing in mice.

Effective reepithelialization after injury is essential for correct wound healing. The upregulation of keratinocyte alpha3beta1 integrin during reepithelialization suggests that this adhesion molecule is involved in wound healing; however, its precise role in this process is unknown. We have shown here that retarded reepithelialization in Itga3(-/-) mouse skin wounds is due predominantly to repressed TGF-beta1-mediated responses. Specifically, expression of the inhibitor of TGF-beta1-signaling Smad7 was elevated in Itga3(-/-) keratinocytes. Indeed, in vivo blockade of Smad7 increased the rate of reepithelialization in Itga3(-/-) and WT wounds to similar levels. Our data therefore indicate that the function of alpha3beta1 integrin as a mediator of keratinocyte migration is not essential for reepithelialization but suggest instead that alpha3beta1 integrin has a major new in vivo role as an inhibitor of Smad7 during wound healing. Moreover, our study may identify a previously undocumented function for Smad7 as a regulator of reepithelialization in vivo and implicates Smad7 as a potential novel target for the treatment of cutaneous wounds.

Real-world experience with pro re nata dosing of intravitreal dexamethasone implant for eyes with refractory diabetic macular edema.

Aims: To evaluate treatment outcomes of pro re nata dosing of intravitreal dexamethasone implant in eyes with refractory diabetic macular edema (DME) amongst Indian subjects. Methods and material: Retrospective, interventional case series. Medical records of 28 eyes of 23 patients with refractory DME who underwent intravitreal dexamethasone (700 µ) implant were reviewed. Paired t-test was carried out to measure mean change in the parameters evaluated. Mann-Whitney U test and Fisher's exact t-test were done to explore differences between groups receiving single or multiple injections. Results: Best corrected visual acuity (BCVA) and central macular thickness (CMT) at baseline were 0.85 (±0.44) and 612 µm (±123), respectively. Mean CMT over 6 months (measured monthly) following injection was 340±119 µm (p=0.001), 346±150 µm (p=0.02), 368±169 µm (p=0.02), 304±174 µm (p=0.001), 525±216 µm (p=0.94) and 532±201 µm (p=0.46), respectively. Mean BCVA at each month following injection was 0.68±0.36 (p=0.02), 0.75±0.45 (p=0.42), 0.55±0.40 (p=0.11), 0.63±0.40 (p=0.12), 0.78±0.30 (p=0.90) and 0.60±0.47 (p=0.92), respectively. Mean follow-up was 12 months (range: 6-33 months). Mean BCVA and CMT at mean 12 months were 0.72±0.46 (p=0.10) and 358 µm±189 (p=0.0001), respectively. Seven eyes had raised IOP; five eyes required cataract extraction. Conclusions: Intravitreal dexamethasone implant is effective in treatment of refractory DME. However, its therapeutic effect lasts for about 4 months.

IL-12 delivered intratumorally by multilamellar liposomes reactivates memory T cells in human tumor microenvironments.

Using a novel loading technique, IL-12 is reported here to be efficiently encapsulated within large multilamellar liposomes. The preclinical efficacy of the cytokine loaded liposomes to deliver IL-12 into human tumors and to reactive tumor-associated T cells in situ is tested using a human tumor xenograft model. IL-12 is released in vivo from these liposomes in a biologically active form when injected into tumor xenografts that are established by the subcutaneous implantation of non-disrupted pieces of human lung, breast or ovarian tumors into immunodeficient mice. The histological architecture of the original tumor tissue, including tumor-associated leukocytes, tumor cells and stromal cells is preserved anatomically and the cells remain functionally responsive to cytokines in these xenografts. The local and sustained release of IL-12 into the tumor microenvironment reactivates tumor-associated quiescent effector memory T cells to proliferate, produce and release IFN-gamma resulting in the killing of tumor cells in situ. Very little IL-12 is detected in the serum of mice for up to 5 days after an intratumoral injection of the IL-12 liposomes. We conclude that IL-12 loaded large multilamellar liposomes provide a safe method for the local and sustained delivery of IL-12 to tumors and a therapeutically effective way of reactivating existing tumor-associated T cells in human solid tumor microenvironments. The potential of this local in situ T cell re-stimulation to induce a systemic anti-tumor immunity is discussed.

Tumor cell alpha3beta1 integrin and vascular laminin-5 mediate pulmonary arrest and metastasis.

Arrest of circulating tumor cells in distant organs is required for hematogenous metastasis, but the tumor cell surface molecules responsible have not been identified. Here, we show that the tumor cell alpha3beta1 integrin makes an important contribution to arrest in the lung and to early colony formation. These analyses indicated that pulmonary arrest does not occur merely due to size restriction, and raised the question of how the tumor cell alpha3beta1 integrin contacts its best-defined ligand, laminin (LN)-5, a basement membrane (BM) component. Further analyses revealed that LN-5 is available to the tumor cell in preexisting patches of exposed BM in the pulmonary vasculature. The early arrest of tumor cells in the pulmonary vasculature through interaction of alpha3beta1 integrin with LN-5 in exposed BM provides both a molecular and a structural basis for cell arrest during pulmonary metastasis.

Exosomes Associated with Human Ovarian Tumors Harbor a Reversible Checkpoint of T-cell Responses.

Nano-sized membrane-encapsulated extracellular vesicles isolated from the ascites fluids of ovarian cancer patients are identified as exosomes based on their biophysical and compositional characteristics. We report here that T cells pulsed with these tumor-associated exosomes during TCR-dependent activation inhibit various activation endpoints including translocation of NFκB and NFAT into the nucleus, upregulation of CD69 and CD107a, production of cytokines, and cell proliferation. In addition, the activation of virus-specific CD8+ T cells that are stimulated with the cognate viral peptides presented in the context of class I MHC is also suppressed by the exosomes. The inhibition occurs without loss of cell viability and coincidentally with the binding and internalization of these exosomes. This exosome-mediated inhibition of T cells was transient and reversible: T cells exposed to exosomes can be reactivated once exosomes are removed. We conclude that tumor-associated exosomes are immunosuppressive and represent a therapeutic target, blockade of which would enhance the antitumor response of quiescent tumor-associated T cells and prevent the functional arrest of adoptively transferred tumor-specific T cells or chimeric antigen receptor T cells. Cancer Immunol Res; 6(2); 236-47. ©2018 AACR.

Role of high-resolution computerized tomography chest in identifying tubercular etiology in patients diagnosed as Eales' disease.

BACKGROUND: The high resolution computerized tomography of chest is an important diagnostic imaging tool to identify any pulmonary tubercular lesion. It's role in Eales' disease to identify any possible association with pulomonary tuberculosis has not been studied earlier. So, this study was conducted to assess the role of high resolution computerized tomography (HRCT) chest in identifying tuberculous etiology in Eales' disease. RESULTS: It was a retrospective study conducted at a tertiary care eye hospital in South India between January 2009 and October 2014 were included. A total of 29 diagnosed cases of Eales' (24 male and 5 female) were included in the study. These patients were followed up for a mean period of 739.75 days. Out of them, 13 (44.8%) had bilateral and 16 (55.2%) had unilateral ocular involvement. Eight cases (34.5%) patients had vitreous inflammation. Mantoux test was positive in 12 (41.4%) cases and chest x-ray suggestive of TB was present in four cases (13.8%). QuantiFERON TB gold was positive in 15 (51.7%) and HRCT chest suggestive of TB was positive in 15 (51.7%) case. Out of 15 Eales' cases with positive HRCT scan suggestive of TB, the commonly noted lesions were calcified nodules 34.5%, mediastinal hilar lymphadenopathy 13.8%, parenchymal soft tissue lesions in 3.4%. Five (17.2%) cases underwent pars plana vitrectomy for non resolving vitreous hemorrhage and one case underwent retinal attachment surgery with encirclage. Six patients were started on 9 months regimen of ATT by the chest physician. Final visual outcome improved in 17(40.5%) eyes, maintained in 21(50%) eyes but vision deterioration in 7(16.7%) eyes. CONCLUSIONS: HRCT chest is an important diagnostic tool to rule out pulmonary tuberculosis in Eales' disease.

Extracellular Vesicles Present in Human Ovarian Tumor Microenvironments Induce a Phosphatidylserine-Dependent Arrest in the T-cell Signaling Cascade.

The identification of immunosuppressive factors within human tumor microenvironments, and the ability to block these factors, would be expected to enhance patients' antitumor immune responses. We previously established that an unidentified factor, or factors, present in ovarian tumor ascites fluids reversibly inhibited the activation of T cells by arresting the T-cell signaling cascade. Ultracentrifugation of the tumor ascites fluid has now revealed a pellet that contains small extracellular vesicles (EV) with an average diameter of 80 nm. The T-cell arrest was determined to be causally linked to phosphatidylserine (PS) that is present on the outer leaflet of the vesicle bilayer, as a depletion of PS-expressing EV or a blockade of PS with anti-PS antibody significantly inhibits the vesicle-induced signaling arrest. The inhibitory EV were also isolated from solid tumor tissues. The presence of immunosuppressive vesicles in the microenvironments of ovarian tumors and our ability to block their inhibition of T-cell function represent a potential therapeutic target for patients with ovarian cancer.

Estrogen promotes ER-negative tumor growth and angiogenesis through mobilization of bone marrow-derived monocytes.

Estrogen has a central role in the genesis and progression of breast cancers whether they are positive or negative for the estrogen receptor (ER). While therapies that disrupt estrogen biosynthesis or ER activity can treat these diseases in postmenopausal women, in younger women where ovarian function remains intact, these anti-estrogen therapies are not as effective. Moreover, emerging clinical evidence suggests that estrogen may promote other cancers. Thus, circulating estrogens may participate in cancer pathogenesis in ways that are not yet understood. In this study, we show that estrogen can promote the outgrowth of murine xenograft tumors established from patient-derived ER-negative breast cancer cells by influencing the mobilization and recruitment of a proangiogenic population of bone marrow-derived myeloid cells. ERα expression was necessary and sufficient in the bone marrow-derived cells themselves to promote tumor formation in response to estrogen. Our findings reveal a novel way in which estrogen promotes tumor formation, with implications for the development and application of anti-estrogen therapies to treat cancer in premenopausal women.

Physicochemical properties of epidermal growth factor receptor inhibitors and development of a nanoliposomal formulation of gefitinib.

Inhibitors of epidermal growth factor (EGF) receptor (EGFR) tyrosine kinases show efficacy in cancers that are highly addicted to nonmutated EGF signaling, but off-target effects limit therapy. Carrier-based formulations could reduce drug deposition in normal tissues, enhance tumor deposition, and reduce free drug concentrations, thereby reducing the side effects. Therefore, the feasibility of developing nanoliposomal formulations of EGFR inhibitors was investigated. Gefitinib and erlotinib fluorescence was characterized as a tool for formulation development. Peak excitation was 345 nm and peak emission was 385-465 nm, depending upon the environment polarity. Emission was negligible in water but intense in nonpolar solvents, membranes, or bound to serum proteins. Cellular uptake and distribution could also be imaged by fluorescence in drug-resistant tumor spheroids. Gefitinib fluorescence characteristics enabled facile optimization of formulations. Although 4-6 mol % gefitinib could be incorporated in the liposome bilayer, 40-60 mol % could be encapsulated in stable, remote-loaded liposomes consisting of distearoylphosphatidylcholine-polyethylene glycol-distereoylphosphatidylethanolamine-cholesterol (9:1:5 mol:mol:mol). Drug leakage in serum, monitored by fluorescence, was minimal over 24 h at 37°C. The results provide both promising lead formulations as well as novel tools for evaluating new formulations of structurally similar receptor tyrosine kinase inhibitors and their cellular uptake and tissue biodistribution.

Functional heterogeneity of breast fibroblasts is defined by a prostaglandin secretory phenotype that promotes expansion of cancer-stem like cells.

Fibroblasts are important in orchestrating various functions necessary for maintaining normal tissue homeostasis as well as promoting malignant tumor growth. Significant evidence indicates that fibroblasts are functionally heterogeneous with respect to their ability to promote tumor growth, but markers that can be used to distinguish growth promoting from growth suppressing fibroblasts remain ill-defined. Here we show that human breast fibroblasts are functionally heterogeneous with respect to tumor-promoting activity regardless of whether they were isolated from normal or cancerous breast tissues. Rather than significant differences in fibroblast marker expression, we show that fibroblasts secreting abundant levels of prostaglandin (PGE2), when isolated from either reduction mammoplasty or carcinoma tissues, were both capable of enhancing tumor growth in vivo and could increase the number of cancer stem-like cells. PGE2 further enhanced the tumor promoting properties of fibroblasts by increasing secretion of IL-6, which was necessary, but not sufficient, for expansion of breast cancer stem-like cells. These findings identify a population of fibroblasts which both produce and respond to PGE2, and that are functionally distinct from other fibroblasts. Identifying markers of these cells could allow for the targeted ablation of tumor-promoting and inflammatory fibroblasts in human breast cancers.

Use of a folding model and in situ spectroscopic techniques for rational formulation development and stability testing of monoclonal antibody therapeutics.

Aggregation is a critical issue that hampers the development of monoclonal antibody therapeutics (Mabs). Traditionally, aggregation is considered a process in which native forms of proteins are transformed into an unstable highly associated form through an intermediate formation step. Here we describe the unfolding of an antiCD40 antibody using a folding model based on Lumry-Eyring nucleated polymerization (LENP) model. This model captures several experimental features of the thermal unfolding of this protein as studied by common in situ biophysical techniques such as circular dichroism, fluorescence spectroscopy, and turbidity measurements. According to this model, the unfolding and aggregation of the antiCD40 antibody is determined by several distinct steps that include conformational change(s) to generate aggregation prone states, reversible oligomer formation, nucleation and growth as well as their kinetics, and the formation of higher order assemblies/aggregates. Furthermore, the loss of monomer is controlled by both thermodynamic (equilibrium unfolding) and kinetic determinants of the unfolding process. This approach captures both of these rate-limiting steps. It can be concluded that this approach is sensitive to formulation conditions such as protein concentration, changes in buffer conditions, and temperature stress. The potential use of this approach in formulation development and stability testing of Mabs is discussed.

Integrin alpha3beta1 potentiates TGFbeta-mediated induction of MMP-9 in immortalized keratinocytes.

Transforming growth factor-beta (TGFbeta) signaling pathways regulate a number of keratinocyte functions during epidermal carcinogenesis and wound healing, including proliferation, survival, and migration. TGFbeta can induce expression of the matrix metalloproteinase-9 (MMP-9), which has critical roles in promoting extracellular matrix remodeling and angiogenesis during tumorigenesis and tissue repair. Integrin alpha3beta1 is a cell adhesion receptor for laminin-332/laminin-5 with important roles in the survival and motility of epidermal keratinocytes. We previously reported that alpha3beta1 induces the expression of MMP-9 in immortalized keratinocytes. In this study, we show that endogenous TGFbeta is required for maximal MMP-9 expression, and that alpha3beta1 is required for full induction of MMP-9 protein and mRNA in response to TGFbeta. This regulation was not observed in non-immortalized, primary keratinocytes, indicating that coordinate regulation of MMP-9 by alpha3beta1 and TGFbeta is a property of immortalized cells. Alpha3beta1 did not regulate endogenous TGFbeta gene expression, TGFbeta bioavailability, or TGFbeta-Smad signaling. However, the combined inductive effects of TGFbeta and alpha3beta1 on MMP-9 were suppressed by a Src family kinase (SFK) inhibitor, indicating involvement of a SFK pathway. These findings provide early evidence of a role for alpha3beta1 in augmenting TGFbeta-mediated induction of MMP-9 in immortalized or transformed keratinocytes during skin carcinogenesis.

Alpha3beta1 integrin regulates MMP-9 mRNA stability in immortalized keratinocytes: a novel mechanism of integrin-mediated MMP gene expression.

Matrix metalloproteinases facilitate cell migration and tumor invasion through their ability to proteolyse the extracellular matrix. The laminin-binding integrin alpha3beta1 is expressed at high levels in squamous cell carcinomas and in normal keratinocytes during cutaneous wound healing. We showed previously that alpha3beta1 is required for MMP-9/gelatinase B secretion in immortalized mouse keratinocytes (MK cells) and that this regulation was acquired as part of the immortalized phenotype, suggesting a possible role for alpha3beta1 during malignant conversion. In the current study, we identify a novel mechanism whereby alpha3beta1 regulates the induction of MMP-9 expression that occurs in response to activation of a MAPK kinase (MEK)/extracellular signal-regulated kinase (ERK) pathway. Inhibition of MEK/ERK signaling in wild-type MK cells with a pharmacological inhibitor, U0126, showed that ERK activation was necessary for high levels of endogenous MMP-9 gene expression and activity of a transfected MMP-9 promoter. Furthermore, activation of MEK/ERK signaling in these cells with an oncogenic mutant of Ras, RasV12, increased both endogenous MMP-9 gene expression and MMP-9 promoter activity. Experiments with alpha3beta1-deficient MK cells revealed that alpha3beta1 was required for both baseline levels and RasV12-induced levels of MMP-9 mRNA expression. However, alpha3beta1 was not required for RasV12-mediated activation of ERK or for ERK-dependent MMP-9 promoter activity. Direct comparison of mRNA turnover in the wild type and alpha3-null MK cells identified a requirement for alpha3beta1 in stabilization of MMP-9 mRNA transcripts. These results identify a novel function for integrins in promoting mRNA stability as a mechanism to potentiate MAPK-mediated gene expression. They also suggest a role for alpha3beta1 in maintaining high levels of MMP-9 mRNA expression in response to oncogenic activation of MEK/ERK signaling pathways.

Alpha 3 beta 1 integrin promotes keratinocyte cell survival through activation of a MEK/ERK signaling pathway.

Inadequate or inappropriate adhesion of epithelial cells to extracellular matrix leads to a form of apoptosis known as anoikis. During various tissue remodelling events, such as wound healing or carcinoma invasion, changes in the physical properties, and/or composition of the extracellular matrix, can lead to anoikis of epithelial cells that lack appropriate receptor-matrix interactions. Laminin-5 is the major ligand for keratinocyte adhesion in the epidermis, and it also promotes keratinocyte survival in vivo and in vitro. Integrins alpha 3 beta 1 and alpha 6 beta 4 are the major receptors for laminin-5; however, specific roles for these integrins in keratinocyte survival have not been determined. In the current study, we exploited keratinocyte cell lines derived from wild-type or alpha 3 integrin knockout mice to reveal a critical role for alpha 3 beta 1 in protecting keratinocytes from apoptosis upon serum withdrawal. We show that alpha 3 beta 1-mediated adhesion to laminin-5 extracellular matrix inhibits proteolytic activation of caspase-3 and TUNEL-staining, both hallmarks of apoptosis. We also show that alpha 3 beta1-mediated adhesion activates focal adhesion kinase (FAK) and extracellular signal-regulated kinase (ERK), and that inhibition of either FAK or ERK signaling leads to apoptosis of keratinocytes attached to laminin-5. alpha 6 beta 4-mediated adhesion to laminin-5 only partially protects cells from apoptosis in the absence of alpha 3 beta 1, and alpha 6 beta 4 is not necessary for cell survival in the presence of alpha 3 beta 1. These results suggest that alpha 3 beta 1 is necessary and sufficient for maximal keratinocyte survival on laminin-5. We propose a model to address the potential importance of alpha 3 beta 1-mediated survival for migrating keratinocytes at the leading edge of a cutaneous wound.