CD44 variant isoforms control experimental autoimmune encephalomyelitis by affecting the lifespan of the pathogenic T cells.

CD44 variant (CD44(v)) isoforms play important roles in the development of autoimmune disorders, including colitis and arthritis, but their role in multiple sclerosis (MS) has been explored only to a limited extent. We determined the functional relevance of CD44(v) isoforms in MS and its animal model, experimental autoimmune encephalomyelitis (EAE). Genetic ablation of CD44(v7) and CD44(v10) isoforms significantly reduced the clinical EAE burden, as well as the number of inflammatory infiltrates. CD44(v7) and CD44(v10) expression on both memory T and antigen-presenting cells, participated in the development of adoptive transfer EAE. Significantly reduced mRNA expression of Th1 signature genes was detected in the brains of CD44(v10-/-) mice compared with those of CD44(WT) mice. Furthermore, forkhead transcription factor 3 (Foxp3), Bcl-2, and inducible nitric oxide synthase (iNOS) levels were reduced in CD44(v10-/-) brains, whereas active caspase-3 was elevated. Brain-infiltrating CD4(hi)CD44(v10+) T cells preceded EAE onset and paralleled disease severity in wild-type but not in CD44(v7-/-) and CD44(v10-/-) mice. CD44(v7) and CD44(v10) expression contributed to EAE by increasing the longevity of autoreactive CD4(hi)panCD44(hi) T cells. Accordingly, the absence of CD44(v7) and CD44(v10) led to increased apoptosis in the inflammatory infiltrates and reduced Th1 responses, resulting in marked disease reduction. Although absent in noninflamed human brains, we detected CD44(v3), CD44(v7), and CD44(v10) isoforms on glial cells and on perivascular infiltrating cells of MS lesions. We conclude that CD44(v7) and CD44(v10), expressed on autoreactive CD4(hi)panCD44(hi) T cells, are critically involved in the pathogenesis of classic EAE by increasing their life span. Targeting these short CD44(v) isoform regions may reduce inflammatory processes and clinical symptoms in MS.

In situ RHAMM protein expression in acute myeloid leukemia blasts suggests poor overall survival.

Treatment options for patients with high-risk acute myeloid leukemia (AML) include high-dose chemotherapy regimens in combination with allogeneic hematopoietic stem cell transplantation, which takes advantage of the donor T-cell-mediated graft-versus-leukemia effect. Together with beneficial responses observed in assays targeted at leukemia-associated antigens (LAA), this encouraged research on cancer vaccines and adoptive cellular therapies in AML. The receptor for hyaluronic acid-mediated motility (RHAMM, CD168) was identified as one of the most promising LAA in AML. Thus far, little is known about in situ expression in leukemic bone marrow blasts or the prognostic role of RHAMM and its interaction partners in AML. We immunohistochemically analyzed the expression and prognostic significance of RHAMM on trephine bone marrow biopsies from 71 AML cases that had been evaluated for cytogenetics and presence of FLT3-internal tandem duplications and NPM1 mutations. Fifty-five patients (77%) were treated with curative intent, while 16 (23%) received the most appropriate supportive care. Twenty of 71 (28%) AML cases were considered RHAMM+. Receiver operating characteristic curves showed significant discriminatory power considering overall survival (OS) in AML patients treated curatively for RHAMM (p = 0.015). Multivariable analysis revealed that expression of RHAMM in >5% of leukemic blasts identifies a subgroup of curatively treated cases with adverse OS independent of failures to achieve complete remission. RHAMM not only represents a promising LAA with specific T-cell responses in AML but, if assessed in situ on blasts, also a probable prognostic factor.

Osteopontin as two-sided mediator of intestinal inflammation.

Osteopontin (OPN) is characterized as a major amplifier of Th1-immune responses. However, its role in intestinal inflammation is currently unknown. We found considerably raised OPN levels in blood of wild-type (WT) mice with dextran sodium sulfate (DSS)-induced colitis. To identify the role of this mediator in intestinal inflammation, we analysed experimental colitis in OPN-deficient (OPN(-/-)) mice. In the acute phase of colitis these mice showed more extensive colonic ulcerations and mucosal destruction than WT mice, which was abrogated by application of soluble OPN. Within the OPN(-/-) mice, infiltrating macrophages were not activated and showed impaired phagocytosis. Reduced mRNA expression of interleukin (IL)-1 beta and matrix metalloproteinases was found in acute colitis of OPN(-/-) mice. This was associated with decreased blood levels of IL-22, a Th17 cytokine that may mediate epithelial regeneration. However, OPN-(/-) mice showed increased serum levels of tumour necrosis factor (TNF)-alpha, which could be due to systemically present lipopolysaccharide translocated to the gut. In contrast to acute colitis, during chronic DSS-colitis, which is driven by a Th1 response of the lamina propria infiltrates, OPN(-/-) mice were protected from mucosal inflammation and demonstrated lower serum levels of IL-12 than WT mice. Furthermore, neutralization of OPN in WT mice abrogated colitis. Lastly, we demonstrate that in patients with active Crohn's disease OPN serum concentration correlated significantly with disease activity. Taken together, we postulate a dual function of OPN in intestinal inflammation: During acute inflammation OPN seems to activate innate immunity, reduces tissue damage and initiates mucosal repair whereas during chronic inflammation it promotes the Th1 response and strengthens inflammation.

Systematic assessment of the prognostic impact of membranous CD44v6 protein expression in colorectal cancer.

AIMS: To assess systematically the membranous expression of CD44v6 in colorectal cancer by immunohistochemistry to determine its prognostic impact, the differential expression between primary and metastatic tumours and expression differences between the tumour centre and invasive front. METHODS AND RESULTS: Immunohistochemistry was performed for CD44v6 on two tissue microarrays. The first included 1279 colorectal tumours with full clinicopathological data. The second consisted of 50 matched primary and metastatic tumours sampled from the tumour centre and the invasive margin. A scoring system was tested by multiple observers. Receiver-operating characteristic curve analysis was used for cut-off point determination. Loss of membranous CD44v6 was associated with pT stage (P = 0.016; sensitivity 85.8%, specificity 20.1%), lymph node metastasis (P = 0.015; sensitivity 52.8%, specificity 55%), an infiltrating tumour margin (P < 0.001; sensitivity 71.4%, specificity 40%) and adverse prognosis (P = 0.011; hazard ratio 0.79, 95% confidence interval 0.7, 0.9), but was not an independent prognostic factor on multivariable analysis. Loss of expression occurred at the invasive front in both primary and metastatic lesions (P < 0.001). CONCLUSIONS: This study outlines an approach to help standardize the immunohistochemical evaluation of CD44v6 and similar markers in colorectal cancer and highlights a significant role for loss of membranous CD44v6 expression in colorectal cancer progression and prognosis.

Overexpression of Specific CD44 Isoforms Is Associated with Aggressive Cell Features in Acquired Endocrine Resistance.

While endocrine therapy is the mainstay of ER+ breast cancer, the clinical effectiveness of these agents is limited by the phenomenon of acquired resistance that is associated with disease relapse and poor prognosis. Our previous studies revealed that acquired resistance is accompanied by a gain in cellular invasion and migration and also that CD44 family proteins are overexpressed in the resistant phenotype. Given the association of CD44 with tumor progression, we hypothesized that its overexpression may act to promote the aggressive behavior of endocrine-resistant breast cancers. Here, we have investigated further the role of two specific CD44 isoforms, CD44v3 and CD44v6, in the endocrine-resistant phenotype. Our data revealed that overexpression of CD44v6, but not CD44v3, in endocrine-sensitive MCF-7 cells resulted in a gain in EGFR signaling, enhanced their endogenous invasive capacity, and attenuated their response to endocrine treatment. Suppression of CD44v6 in endocrine-resistant cell models was associated with a reduction in their invasive capacity. Our data suggest that upregulation of CD44v6 in acquired resistant breast cancer may contribute to a gain in the aggressive phenotype of these cells and loss of endocrine response through transactivation of the EGFR pathway. Future therapeutic targeting of CD44v6 may prove to be an effective strategy alongside EGFR-targeted agents in delaying/preventing acquired resistance in breast cancer.

Enhanced cell surface CD44 variant (v6, v9) expression by osteopontin in breast cancer epithelial cells facilitates tumor cell migration: novel post-transcriptional, post-translational regulation.

Osteopontin (OPN) is a glycosylated, secreted phosphoprotein that functions both as a cell attachment and chemotactic factor. Elevated expression of OPN confers enhanced metastatic ability on transformed cells, suggesting that OPN may contribute to the malignant progression of tumors. Migration of mammary carcinoma cells is stimulated by OPN via interactions with integrins and CD44 cell surface receptors. We hypothesized that OPN modulates specific CD44 isoform expression to facilitate breast cancer cell migration. The 21NT tumorigenic human breast cancer cell line was examined for regulation of CD44 expression at both the mRNA and protein levels in response to an engineered increase in OPN expression under CMV promoter control. Significant up-regulation of CD44s isoform mRNA expression was observed, but no change in CD44v6, v8, v9 or v10 mRNA levels. While there were elevated levels of CD44s, v6 and v9 protein at the cell surface, at the level of total cellular protein only CD44s and v6 were markedly increased. This suggests that OPN can regulate CD44 expression at both transcriptional and post-transcriptional (both amount and localization of protein) levels. To validate the functional consequence of OPN regulation of CD44 expression, we demonstrate that OPN-mediated cell migration was reduced by exposure to a anti-pan CD44 antibody, and to anti-CD44v6 and anti-CD44v9 function-blocking antibodies. Our data provide evidence that in 21NT cells OPN enhances CD44s mRNA expression, increases cell surface expression of CD44 variant forms without a change in mRNA levels, and stimulates cell migration.

Allogeneic reconstitution after nonmyeloablative conditioning: mitigation of graft-versus-host and host-versus-graft reactivity by anti-CD44v6.

T-cell maturation is accelerated in transgenic mice expressing rat CD44v4-v7 on T cells, the effect being blocked by anti-CD44v6. This finding suggested functional activity of CD44v6 in thymocyte development. We tested the hypothesis by antibody blocking and using mice with targeted deletion of CD44v6/v7 exon products (CD44v6/v7(-/-)). When lethally irradiated CD44v6/v7-competent (CD44v6/v7(+/+)) mice were reconstituted syngeneically, higher numbers of CD44v6/v7(-/-) than CD44v6/v7(+/+) BMC were required for survival, the period of reconstitution was prolonged, and regain of immunocompetence was delayed. Similar findings were observed in lethally irradiated, anti-CD44v6-treated syngeneic CD44v6/v7(+/+) hosts. Thus, CD44v6/v7 supports maturation and expansion of hematopoietic progenitor cells. Surprisingly, reconstitution with CD44v6/v7(-/-) BMC or anti-CD44v6 treatment of the nonlethally irradiated allogeneic CD44v6/v7(+/+) host had only a minor impact on survival rates. When nonlethally irradiated CD44v6/v7(-/-) hosts received an allogeneic graft, survival rates were improved. These phenomena have been a result of reduced GvH reactivities when the donor was CD44v6/v7(-/-) and reduced HvG reactivities in the CD44v6/v7(-/-) host. Thus, although a deficit or blockade of CD44v6/v7 has a negative impact on hematopoietic reconstitution, a transient blockade will be of benefit for the allogeneically reconstituted host because of a strong reduction in GvH and HvG reactivities.

Elevated levels of soluble CD44 are associated with advanced disease and in vitro proliferation of neoplastic lymphocytes in B-cell chronic lymphocytic leukaemia.

PURPOSE: Increased expression of the adhesion molecule CD44 has been associated with an unfavourable clinical outcome in lymphomas. We evaluated the prognostic value of soluble CD44 in B-cell chronic lymphocytic leukaemia (B-CLL) and analysed the source and regulation of CD44 secretion in B-CLL clones in vitro. PATIENTS AND METHODS: Levels of soluble CD44 standard (sCD44s) and of the soluble variant isoform CD44v6 (sCD44v6) were analysed by enzyme linked immuno sorbent assay. Highly purified B-CLL cells (98% CD19 + CD3 - cells) were stimulated in vitro by different combinations of thioredoxin (Trx), Staphylococcus aureus Cowan strain 1 (SAC), IL-2, IL-4, IL-10, 12-O-tetradecanoyl-phorbol-13-acetate (TPA) and by anti-CD40 mAbs presented on irradiated CD32L cells. RESULTS: Serum levels of sCD44s and of sCD44v6 are significantly elevated in B-CLL patients (n = 90) in comparison with normal persons (n = 44) (P < 0.001). Elevated levels of sCD44s and sCD44v6 are associated with an advanced disease as reflected by an extended lymph node involvement (P < 0.02), an advanced Binet (P < 0.03) and Rai stage (P < 0.04) and chemotherapy requirement (P < 0.02). High levels of sCD44s are associated with high leukocyte counts (P < 0.04) and increased sCD44v6 is significantly associated with splenomegaly (P < 0.002). In B-CLL sCD44s as well as sCD44v6 is shed from leukaemia cells as shown by in vitro cultures. Stimulation of B-CLL clones results in a proliferation-associated increased secretion of sCD44s (rho = 0.7; P = 0.0001) and of sCD44v6 (rho = 0.5; P = 0.005). B-CLL clones from advanced stage patients are characterised by an increased capacity for proliferation and CD44 production in comparison with early stage patients. CONCLUSIONS: Both sCD44s and sCD44v6 represent a reliable prognostic marker in B-CLL and may be involved in the pathogenesis of B-CLL.

Signaling via the AHR leads to enhanced usage of CD44v10 by murine fetal thymic emigrants: possible role for CD44 in emigration.

Signaling via the endogenous arylhydrocarbon receptor (AHR) affects proliferation, differentiation, function and gene expression of thymocytes. In the present study, we show that treatment of mouse fetal thymus lobes in organ culture (FTOC) with AHR ligands results in (a) a drastic decrease in the emigration of thymocytes in terms of numbers and types of cells, and (b) preferential emigration of CD4-CD8- (DN) cells expressing CD44v7- and CD44v10-containing isoforms on the cell surface. Moreover, a higher level of transcripts of various other CD44 variant isoforms (CD44v) could be detected by RT-PCR in emigrants from fetal thymi exposed to either AHR-agonist during culture. Expression of CD44v9-10-containing isoforms could be exclusively detected in DN thymic emigrants. Thus, signaling via AHR by ligands alters CD44v expression patterns in a thymocyte subpopulation. Furthermore, emigration could be decreased by the addition of anti-panCD44 antibodies to TCDD-treated FTOCs, suggesting a role for CD44 in emigration.

Coexpression of CD44 variant isoforms and receptor for hyaluronic acid-mediated motility (RHAMM, CD168) is an International Prognostic Index and C-MYC gene status-independent predictor of poor outcome in diffuse large B-cell lymphomas.

OBJECTIVE: Expression of CD44 variant (v) isoforms substantiates poor prognosis in patients with hematological malignancies. We have previously shown that CD44v6 expression in diffuse large B-cell lymphoma (DLBCL) correlates with advanced disease stage and is predominantly detected in non-germinal center B-cell-like DLBCL subtypes. With the growing number of associated molecules found to form functional complexes with CD44, we analyzed a larger cohort of cyclophosphamide, doxorubicin, vincristine, prednisone, and equivalently treated DLBCL patients to define the prognostic role of such CD44-associated molecules. MATERIALS AND METHODS: Two-hundred and ninety formalin-fixed, paraffin-embedded primary DLBCL tissue samples were analyzed in tissue microarrays. To obtain potential biologically meaningful associations, optimal cutoff values were established by receiver operating characteristic curves. The prognostic significance of every possible multimarker phenotype was also addressed. RESULTS: We showed that coexpression of any of the CD44v with the receptor for hyaluronic acid-mediated motility (RHAMM, CD168) identifies a subgroup of DLBCL patients with a very poor prognosis, independent of the International Prognostic Index. These patients did not show C-MYC translocations or amplifications. CD44v-RHAMM coexpression was most prevalent in non-germinal center DLBCL cases and usually coincided with expression of osteopontin. CONCLUSION: Evaluation of CD44v-RHAMM coexpression may improve the accuracy of DLBCL prognosis and identify a subgroup of patients who will benefit from therapeutic alternatives to cyclophosphamide, doxorubicin, vincristine, and prednisone.

The CD44 standard/ezrin complex regulates Fas-mediated apoptosis in Jurkat cells.

The transmembrane receptor CD44 conveys important signals from the extracellular microenvironment to the cytoplasm, a phenomena known as "outside-in" signaling. CD44 exists as several isoforms that result from alternative splicing, which differ only in the extracellular domain but yet exhibit different activities. CD44 is a binding partner for the membrane-cytoskeleton cross-linker protein ezrin. In this study, we demonstrate that only CD44 standard (CD44s) colocalizes and interacts with the actin cross-linkers ezrin and moesin using well-characterized cell lines engineered to express different CD44 isoforms. Importantly, we also show that the association CD44s-ezrin-actin is an important modulator of Fas-mediated apoptosis. The results highlight a mechanism by which signals from the extracellular milieu regulate intracellular signaling activities involved in programmed cell death.

The osteopontin - CD44 pathway is superfluous for the development of autoimmune myocarditis.

Osteopontin (OPN) and CD44 have been implicated in the development of autoimmune diseases, including arthritis and multiple sclerosis, as well as chronic inflammatory diseases, such as atherosclerosis and colitis. To investigate their roles in autoimmune myocarditis induced by immunization with heart alpha-myosin (MyHC-alpha), a mouse model of human cardiomyopathy, we analyzed mice lacking OPN or CD44v6/v7, a CD44 isoform that binds OPN. Both, OPN(-/-) and CD44v6/v7(-/-) mice developed myocarditis with the same prevalence and severity as BALB/c wild-type controls. Furthermore, treatment of BALB/c mice with a pan-neutralizing anti-CD44 antibody did not affect the disease outcome. Consistently, expansion of MyHC-alpha-specific autoimmune CD4(+) T cells and MyHC-alpha autoantibody responses from either CD44v6/v7(-/-) mice or OPN(-/-) mice was indistinguishable from their wild-type controls. Thus, OPN and CD44v6/v7 are merely spectators rather than protagonists in autoimmune myocarditis.

The involvement of osteopontin and its receptors in multiple myeloma cell survival, migration and invasion in the murine 5T33MM model.

Multiple myeloma (MM) is a malignancy characterised by the accumulation of monoclonal plasma cells in the bone marrow. Different reports indicate the expression of CD44 variant isoforms (CD44v) by MM cells. Osteopontin (OPN), which is expressed by MM cells, is known to be a ligand for CD44v6. In this study, we investigated the role of OPN with emphasis on a functional correlation between OPN and CD44v in the 5T33MM model. Our group reported the expression of CD44v by 5T33MM cells. Using this model, we have demonstrated the secretion of OPN by 5T33MM cells. OPN affected 5T33MM cell survival by increasing proliferation and inhibiting apoptosis. OPN also stimulated 5T33MM cell migration, transendothelial migration and matrix metalloproteinase-9 activity. We confirmed the proliferative and migratory effects of OPN on human MM cells. By applying inhibiting anti-CD44v6 antibodies, we found that OPN stimulated cell proliferation by engaging this isoform. Anti-CD44v antibodies and RGD peptides both inhibited cell migration, suggesting an involvement of both, CD44v isoforms and integrins. In conclusion, OPN may act as a mediator of MM cell survival by engaging CD44v. The protein is further involved in migration and invasion of MM cells through the activation of either alphavbeta3 integrin or CD44v isoforms.

Overexpression of the receptor for hyaluronic acid mediated motility is an independent adverse prognostic factor in colorectal cancer.

RHAMM, a member of the microtubule-associated protein family that interacts with the mitogen-activated protein kinase pathway, is associated with tumor progression, aggressive disease and shortened survival in several tumor types. This study aimed to determine the prognostic value of RHAMM in colorectal cancer (CRC). A series of 1420 unselected, nonconsecutive CRC resections were subdivided into three groups: (1) DNA mismatch repair (MMR)-proficient, (2) MLH1 negative and (3) presumed Lynch syndrome. Immunohistochemical analysis of RHAMM expression (0 vs >0%), increasing expression (increasing percentage positivity) and complete expression (100 vs <100%) was performed using tissue microarray technique and the results were correlated with clinicopathological parameters. Fifty-seven tissue samples of normal colonic mucosa were included as a control group. In a univariate analysis increasing and complete expression of RHAMM were associated with higher N stage (P=0.023 and 0.021) and worse survival (P<0.0001) in MMR-proficient CRC. Complete expression of RHAMM was associated with worse survival in presumed Lynch syndrome (P=0.016). In MLH1-negative CRC there was no association between RHAMM expression and the clinicopathological features. In a multivariate analysis, increasing RHAMM expression was an independent adverse prognostic factor in MMR-proficient CRC (P<0.0001) and complete expression in MMR-proficient CRC and presumed Lynch syndrome (P<0.0001 and P=0.031, respectively). Nuclear pERK expression was associated with increasing RHAMM expression in MMR-proficient CRC (P=0.012) and with complete RHAMM expression in presumed HNPCC (P=0.03). Increasing and complete RHAMM expressions are independent adverse prognostic factors in MMR-proficient CRC and presumed Lynch syndrome.

Role of CD44 and its v7 isoform in staphylococcal enterotoxin B-induced toxic shock: CD44 deficiency on hepatic mononuclear cells leads to reduced activation-induced apoptosis that results in increased liver damage.

Exposure to bacterial superantigens such as staphylococcal enterotoxin B (SEB) leads to the induction of toxic shock syndrome which results in multiorgan failure, including liver damage. In the present study, we investigated the role of CD44 in SEB-induced liver injury. Injection of SEB into d-galactosamine-sensitized CD44 wild-type (WT) mice led to a significant increase in CD44 expression on liver T cells, NK cells, and NKT cells. Administration of SEB to CD44 knockout (KO) mice caused significantly enhanced liver damage which correlated with elevated numbers of T cells, NK cells, NKT cells, and macrophages in the liver and increased production of tumor necrosis factor alpha and gamma interferon compared to CD44 WT mice. Furthermore, liver mononuclear cells from CD44 KO mice were resistant to SEB-induced apoptosis, and cDNA microarray analysis revealed that SEB activation of such cells led to the induction of several antiapoptotic genes and repression of proapoptotic genes. Examination of CD44 isoforms revealed that SEB exposure altered CD44 variant 7 (v7) isoform expression. Interestingly, mice bearing a specific deletion of the CD44v7 exon exhibited increased susceptibility to SEB-induced hepatitis. Finally, treatment of CD44 WT mice with anti-CD44 monoclonal antibodies reduced expression of CD44 in liver mononuclear cells and caused increased susceptibility to SEB-induced liver injury. Together, these data demonstrate that the expression of CD44 and/or CD44v7 on SEB-activated liver mononuclear cells facilitates their rapid apoptosis, thereby preventing severe liver injury in wild-type mice, and suggest that CD44 plays an important role in the regulation and elimination of immune cells in the liver.

Functional involvement of CD44 variant 7 in gut immune response.

A major problem in inflammatory bowel disease (IBD) is the accumulation of highly activated T-helper cells that are refractory to apoptosis induction. Hence, persistent inflammatory lesions are prevalent and are the basis of chronic disease. In IBD upregulation of costimulatory molecules on lamina propria lymphocytes has been described leading to apoptosis resistance. CD44 is a cell adhesion molecule and a signalling receptor that functions as a costimulatory molecule in T-cell activation. Several variant isoforms of CD44 (CD44v) are expressed by alternative splicing of variant exons encoding extracellular regions. Particularly isoforms containing CD44v7 are expressed on T cells and macrophages in T-helper-1 (Th1)-mediated chronic inflammation and autoimmune diseases. In this review recent data on the functional involvement of CD44v7 isoforms in IBD are discussed. In a mouse model of experimental colitis blockade or deletion of CD44v7 protects mice from severe intestinal inflammation by inducing apoptosis in lamina propria mononuclear cells. Recently, we observed that in lamina propria mononuclear cells from the inflamed but not uninflamed mucosa of patients with Crohn's disease, blockade of CD44v7 isoforms also induces apoptosis. The finding that obstruction of CD44v7 isoforms can antagonize Th1-cytokine-dependent immune pathology identifies CD44v7 as a target in the treatment of inflammatory diseases such as IBD, rheumatoid arthritis, multiple sclerosis and other autoimmune diseases in which CD44v7 isoforms are upregulated.

Targeted deletion of CD44v7 exon leads to decreased endothelial cell injury but not tumor cell killing mediated by interleukin-2-activated cytolytic lymphocytes.

In the current study, we investigated the nature and role of CD44 variant isoforms involved in endothelial cell (EC) injury and tumor cell cytotoxicity mediated by IL-2-activated killer (LAK) cells. Treatment of CD44 wild-type lymphocytes with IL-2 led to increased gene expression of CD44 v6 and v7 variant isoforms and to significant induction of vascular leak syndrome (VLS). CD44v6-v7 knockout (KO) and CD44v7 KO mice showed markedly reduced levels of IL-2-induced VLS. The decreased VLS in CD44v6-v7 KO and CD44v7 KO mice did not result from differential activation and expansion of CD8+ T cells, NK, and NK-T cells or from altered degree of perivascular lymphocytic infiltration in the lungs. LAK cells from CD44v7 KO mice showed a significant decrease in their ability to adhere to and mediate lysis of EC but not lysis of P815 tumor cells in vitro. CD44v7-mediated lysis of EC by LAK cells was dependent on the activity of phosphatidylinositol 3-kinase and tyrosine kinases. Interestingly, IL-2-activated LAK cells expressing CD44hi but not CD44lo were responsible for EC lysis. Furthermore, lysis of EC targets could be blocked by addition of soluble or enzymatic cleavage of CD44v6-v7-binding glycosaminoglycans. Finally, anti-CD44v7 mAbs caused a significant reduction in the adherence to and killing of EC and led to suppression of IL-2-induced VLS. Together, this study suggests that the expression of CD44v7 on LAK cells plays a specific role in EC injury and that it may be possible to reduce EC injury but not tumor cell killing by specifically targeting CD44v7.