SummaryAdaptive immunity, a vertebrate specialization, adds memory and exquisite specificity to the basic innate immune responses present in invertebrates while conserving metabolic resources. In adaptive immunity, antigenic challenge requires extremely rapid proliferation of rare antigen-specific lymphocytes to produce large, clonally expanded effector populations that neutralize pathogens. Rapid proliferation and resulting clonal expansion are dependent on CD98, a protein whose well-conserved orthologs appear restricted to vertebrates. Thus, CD98 supports lymphocyte clonal expansion to enable protective adaptive immunity, an advantage that could account for the presence of CD98 in vertebrates. CD98 supports lymphocyte clonal expansion by amplifying integrin signals that enable proliferation and prevent apoptosis. These integrin-dependent signals can also provoke cancer development and invasion, anchorage-independence and the rapid proliferation of tumor cells. CD98 is highly expressed in many cancers and contributes to formation of tumors in experimental models. Strikingly, vertebrates, which possess highly conserved CD98 proteins, CD98-binding integrins and adaptive immunity, also display propensity towards invasive and metastatic tumors. In this Commentary, we review the roles of CD98 in lymphocyte biology and cancer. We suggest that the CD98 amplification of integrin signaling in adaptive immunity provides survival benefits to vertebrates, which, in turn, bear the price of increased susceptibility to cancer.

Proliferation of antigen-specific lymphocytes and resulting clonal expansion is essential for adaptive immunity. We report that B cell-specific deletion of CD98hc reduced antibody responses due to total suppression of B cell proliferation and subsequent plasma cell formation. Deletion of CD98hc didn’t impair early B cell activation, but did inhibit later activation of the MAP kinase Erk1/2 and down regulation of the p27 cell cycle inhibitor. Reconstitution of CD98hc-deficient B cells with CD98hc mutants revealed that the integrin-binding domain of CD98hc is required, but the amino acid transport function of CD98hc is dispensable, for B cell proliferation. Thus, CD98hc supports integrin-dependent rapid proliferation of B cells. We propose that the advantage of adaptive immunity favored appearance of CD98hc in vertebrates.

SUMMARY Acute myelogenous leukemia (AML) is an aggressive disease associated with drug resistance and relapse. To improve therapeutic strategies, it is critical to better understand the mechanisms that underlie AML progression. Here we show that the integrin binding glycoprotein CD98 plays a central role in AML. CD98 promotes AML propagation and lethality by driving engagement of leukemia cells with their microenvironment and maintaining leukemic stem cells. Further, delivery of a humanized anti-CD98 antibody blocks growth of patient-derived AML, highlighting the importance of this pathway in human disease. These findings indicate that microenvironmental interactions are key regulators of AML and that disrupting these signals with inhibitors such as CD98-antibodies may be a valuable therapeutic approach for adults and children with this disease.

CD98hc (CD98 heavy chain, 4F2 antigen, Slc3a2) was discovered as a lymphocyte activation antigen. Deletion of CD98hc in B cells leads to complete failure of B cell proliferation, plasma cell formation, and antibody secretion. Here we examined the role of T cell CD98 in cell-mediated immunity and autoimmune disease pathogenesis by specifically deleting it in murine T cells. Deletion of T cell CD98 prevented experimental autoimmune diabetes associated with dramatically reduced T cell clonal expansion. Nevertheless initial T cell homing to pancreatic islets was unimpaired. In sharp contrast to B cells, CD98-null T cells showed only modestly impaired antigen-driven proliferation and nearly normal homeostatic proliferation. Furthermore, these cells were activated by antigen leading to cytokine production (CD4) and efficient cytolytic killing of targets (CD8). The integrin binding domain of CD98 was necessary and sufficient for full clonal expansion, pointing to a role for adhesive signaling in T cell proliferation and autoimmune disease. When we expanded CD98-null T cells in vitro, they adoptively transferred diabetes, establishing that impaired clonal expansion was responsible for protection from disease. Thus the integrin binding domain of CD98 is required for antigen-driven T cell clonal expansion in the pathogenesis of an autoimmune disease and may represent a useful therapeutic target.

Adoptive transfer of diabetogenic CD4 Th1 T cell clones into young NOD or NOD.scid recipients rapidly induces onset of diabetes and also provides a system for analysis of the pancreatic infiltrate. Although many reports have suggested a role for macrophages in the inflammatory response, there has been little direct characterization of macrophage activity in the pancreas. We showed previously that after migration to the pancreas, diabetogenic CD4 T cell clones produce a variety of inflammatory cytokines and chemokines, resulting in the recruitment of macrophages. In this study, we investigated mechanisms by which macrophages are recruited and activated by T cells. Analysis of infiltrating cells after adoptive transfer by the diabetogenic T cell clone BDC-2.5 indicates that large numbers of cells staining for both F4/80 and CD11b are recruited into the pancreas where they are activated to make IL-1β, TNF-α, and NO, and express the chemokine receptors CCR5, CXCR3, and CCR8. Diabetogenic CD4 T cell clones produce several inflammatory chemokines in vitro, but after adoptive transfer we found that the only chemokine that could be detected ex vivo was CCL1. These results provide the first evidence that CCR8/CCL1 interaction may play a role in type 1 diabetes through macrophage recruitment and activation.

Integrins are adhesion receptors important for hematopoiesis, leukocyte trafficking, and formation of immunological synapses; hence, they may provide targets for therapeutic intervention in leukocyte-driven pathologies. Blocking integrin-ligand binding is one strategy for inhibiting integrins; however, a complete loss of integrin function can lead to mechanism-based toxicities. Because integrin alpha and beta subunits interact with a variety of other proteins to receive and transmit cellular signals, targeting these integrin-associated proteins may utilize alternative sites for intervention that lead to therapies with fewer side effects. This review summarizes integrin-associated proteins in leukocytes and focuses on four of these proteins with perceived therapeutic potential. Specific mutations in the alpha4 integrin cytoplasmic tail block or enforce binding to paxillin and thus modulate integrin signaling required for efficient cell migration. Similarly, the association of RAPL(NORE1B) with beta2 integrins may participate in adhesive and migratory events in leukocytes. The beta integrin cytoplasmic tail-binding protein talin is critical for increasing the affinity of integrins (activation), and blockade of talin binding can prevent leukocyte arrest on the endothelium. Finally, the membrane protein CD98 mediates beta1 and beta3 integrin signaling and may be involved in leukocyte functions. Identification of biologically important interactions of integrins and signaling proteins can thus pave the way to new strategies for manipulating leukocyte functions.

Effector function of T cells in autoimmune diabetes has been widely studied with mixed populations of lymphoid T cells stimulated ex vivo, but this approach does not permit evaluation of the contribution by a single T cell clone in the inflammatory site during pathogenesis. We have investigated cytokine production both in vitro and in vivo in a panel of diabetogenic CD4 Th1 T cell clones derived from the NOD mouse. SuperArray analysis showed a common pattern of mRNA expression for inflammatory cytokines and receptors in vitro after TCR stimulation. Ex vivo intracellular cytokine staining demonstrated that two important inflammatory cytokines, IFN-γ and TNF-α, were being made by these T cells recovered from the pancreas 6 days following adoptive transfer. TNF-α produced in the pancreas by pathogenic T cell clones and recruited macrophages was not the membrane-bound form. Secreted TNF-α can lead to production of multiple inflammatory chemokines, as were observed in the pathogenic clones by intracellular cytokine staining. Our results not only define the nature of an inflammatory cytokine response critical to development of diabetes, but also suggest its role in the regulation of other events during pathogenesis induced by CD4 T cells. Similar analyses in other models demonstrated that disease induced by CD4 T cell clones closely resembles spontaneous autoimmune diabetes in which both CD4 and CD8 T cells are required. Thus, cloned T cells in effect amplify effector function of T cells which otherwise may be difficult to detect without ex vivo stimulation.

CD98 is expressed on several tissue types and specifically upregulated on fast-cycling cells undergoing clonal expansion. Various solid (e.g., nonsmall cell lung carcinoma) as well as hematological malignancies (e.g., acute myeloid leukemia) overexpress CD98. We have identified a CD98-specific mouse monoclonal antibody that exhibits potent preclinical antitumor activity against established lymphoma tumor xenografts. Additionally, the humanized antibody designated IGN523 demonstrated robust tumor growth inhibition in leukemic cell-line derived xenograft models and was as efficacious as standard of care carboplatin in patient-derived nonsmall lung cancer xenografts. In vitro studies revealed that IGN523 elicited strong ADCC activity, induced lysosomal membrane permeabilization and inhibited essential amino acid transport function, ultimately resulting in caspase-3 and -7-mediated apoptosis of tumor cells. IGN523 is currently being evaluated in a Phase I clinical trial for acute myeloid leukemia (NCT02040506). Furthermore, preclinical data support the therapeutic potential of IGN523 in solid tumors.CD98 is a heterodimeric protein that comprises a heavy and light chain. The CD98 heavy chain is a type II transmembrane glycoprotein that forms a heterodimer via covalent linkage to one of 6 amino acid transporters. 1,2 CD98 is overexpressed on the cell surface of almost all tumor cells, regardless of tissue origin and increased expression of a CD98-light chain, L-type amino acid transporter 1 (LAT-1) occurs in many types of human cancers, including breast, colon, oral, ovarian, esophageal, glioma and leukemia. 3,4 Increased uptake of amino acids supports the high growth rate of cancer cells by providing the building blocks for protein synthesis. [4][5][6] Moreover, the higher expression of CD98 heavy chain and LAT-1 in metastatic vs. primary tumors suggests that over-expression of CD98/LAT-1 may facilitate progression and metastasis of human cancers.CD98 also regulates integrin signaling by association with integrin b-subunits, thereby controlling cell proliferation, survival, migration, epithelial adhesion and polarity. 3,7 The function of CD98 in regulating both amino acid transport and integrin signaling can contribute to the rapid proliferation and clonal expansion of lymphocytes and tumor cells. 3 The expression pattern and pleiotropic function of CD98 heavy and light chains suggest these proteins are promising targets for treatment of a variety of human cancers. Although small molecule inhibitors of LAT-1 activity have demonstrated preclinical antitumor activity in a number of cancer cell types, including NSCLC, colon cancer, oral cancer and breast cancer, development of antibodies against CD98 heavy chain has received less focus. 5,[8][9][10] Murine monoclonal antibodies to CD98 inhibit lymphocyte proliferation and the growth of bladder cancer, lymphoma, glioma, prostate and colon cancer cells in preclinical models. [11][12][13] To identify therapeutic anti-CD98 antibodies with significant antitumor activity, we ...