We explored the utility of targeting anaplastic lymphoma kinase (ALK), a cell surface receptor overexpressed on pediatric solid tumors, using chimeric antigen receptor (CAR)-based immunotherapy. T cells expressing a CAR incorporating the single-chain variable fragment sequence of the ALK48 mAb linked to a 4-1BB-CD3ζ signaling domain lysed ALK-expressing tumor lines and produced interferon-gamma upon antigen stimulation but had limited anti-tumor efficacy in two xenograft models of human neuroblastoma. Further exploration demonstrated that cytokine production was highly dependent upon ALK target density and that target density of ALK on neuroblastoma cell lines was insufficient for maximal activation of CAR T cells. In addition, ALK CAR T cells demonstrated rapid and complete antigen-induced loss of receptor from the T cell surface via internalization. Using a model that simultaneously modulated antigen density and CAR expression, we demonstrated that CAR functionality is regulated by target antigen and CAR density and that low expression of either contributes to limited anti-tumor efficacy of the ALK CAR. These data suggest that stoichiometric relationships between CAR receptors and target antigens may significantly impact the anti-tumor efficacy of CAR T cells and that manipulation of these parameters could allow precise tuning of CAR T cell activity.
Anaplastic lymphoma kinase (ALK) is a receptor tyrosine kinase that is transiently expressed in specific regions of the central and peripheral nervous systems, suggesting a role in its normal development and function. The nature of the cognate ligands of ALK in vertebrate is still a matter of debate. We produced a panel of monoclonal antibodies (mAbs) directed against the extracellular domain of the human receptor. Two major species of ALK (220 and 140 kDa) were identified in transfected cells, and the use of our mAbs established that the 140-kDa species results from a cleavage of the 220-kDa form. Two mAbs, in the nM range, induced the differentiation of PC12 cells transiently transfected with ALK. In human embryonic kidney 293 cells stably expressing ALK, these two mAbs strongly activated the receptor and subsequently the mitogen-activated protein kinase pathway. We further showed for the first time that activation of ALK also resulted in a specific activation of STAT3. In contrast, other mAbs presented the characteristics of blocking antibodies. Finally, in these cell systems, a mitogenic form of pleiotrophin, a proposed ligand of ALK, failed to activate this receptor. Thus, in the absence of clearly established ligand(s) in vertebrates, the availability of mAbs allowing the activation or the inhibition of the receptor will be essential for a better understanding of the biological roles of ALK. Receptors tyrosine kinase (RTKs)1 play essential roles during the development of the nervous system, regulating a wide range of cellular processes such as proliferation, survival, differentiation, and synaptogenesis. Generally, after ligand binding, RTK dimerizes, autophosphorylates, and initiates signal transduction cascades that subsequently lead to cellular responses (for review, see Ref. 1).Anaplastic lymphoma kinase (ALK) was originally identified as a RTK that acquires transforming capability when truncated and fused in the t(2;5) chromosomal rearrangement associated with the non-Hodgkin lymphoma (2). This translocation produces a fusion gene that encodes a soluble chimeric transforming protein comprising the N-terminal portion of the phosphoprotein nucleophosmin (NPM) linked to the cytoplasmic portion of ALK. It has been demonstrated that the NPM portion is responsible for the dimerization of the fusion protein leading to the constitutive activation of the kinase and to the transforming activity. Phospholipase C␥, phosphatidylinositol 3-kinase, STATs, and Src appear to be important downstream targets of NPM-ALK which contribute to its mitogenic and antiapoptotic activities (3-7). ALK is also involved in different variant chromosomal translocations (for review, see Ref. 8), all leading to the expression of fusion proteins exhibiting a constitutive activation of the kinase.Human, mouse, and Drosophila cDNAs encoding full-length ALK have been characterized (9 -11). The deduced amino acid sequences revealed that ALK is a novel RTK having an extracellular domain, a single transmembrane domain, and an intracellular do...
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