Imaging CAR T-cell kinetics in solid tumors: Translational implications

Abstract

Summary Success in solid tumor chimeric antigen receptor (CAR) T-cell therapy requires overcoming several barriers, including lung sequestration, inefficient accumulation within the tumor, and target-antigen heterogeneity. Understanding CAR T-cell kinetics can assist in the interpretation of therapy response and limitations and thereby facilitate developing successful strategies to treat solid tumors. As T-cell therapy response varies across metastatic sites, the assessment of CAR T-cell kinetics by peripheral blood analysis or a single-site tumor biopsy is inadequate for interpretation of therapy response. The use of tumor imaging alone has also proven to be insufficient to interpret response to therapy. To address these limitations, we conducted dual tumor and T-cell imaging by use of a bioluminescent reporter and positron emission tomography in clinically relevant mouse models of pleural mesothelioma and non-small cell lung cancer. We observed that the mode of delivery of T cells (systemic versus regional), T-cell activation status (presence or absence of antigen-expressing tumor), and tumor-antigen expression heterogeneity influence T-cell kinetics. The observations from our study underscore the need to identify and develop a T-cell reporter—in addition to standard parameters of tumor imaging and antitumor efficacy—that can be used for repeat imaging without compromising the efficacy of CAR T cells in vivo .

A multimodal imaging workflow for monitoring CAR T cell therapy against solid tumor from whole-body to single-cell level

“…Compared to 2D BLI, cognate BLT data facilitated the quantitative and anatomical analysis of the bioluminescent source and thus allowed improved pharmacokinetic evaluation of CAR T cells. Consistent with previous reports, we observed initial lung sequestration of systemically administered CAR T cells 34 , 35 followed by a reduced overall number of CAR T cells entering the circulation as indicated by a whole-body bioluminescent intensity drop between day 0 and 5. Thereafter, antigen-activated and -nonactivated CAR T cells displayed distinct accumulation kinetics: while in the EGFR CAR T cell (± IL-2)-treated cohorts, the first signal emerged at the tumor site, BDCA-2 CAR T cells (± IL-2) primarily homed to the spleen.…”

“…Thereafter, antigen-activated and -nonactivated CAR T cells displayed distinct accumulation kinetics: while in the EGFR CAR T cell (± IL-2)-treated cohorts, the first signal emerged at the tumor site, BDCA-2 CAR T cells (± IL-2) primarily homed to the spleen. Although this observation is in line with several other studies 34 , 58 , 59 , Sellmyer and colleagues reported discrepant findings in which both therapeutic and control T cells initially homed to the spleen followed by an accumulation at the tumor site 27 . This contradiction can be explained by the study of Skovgard and colleagues who demonstrated that CAR T cell accumulation occurs in an antigen-dependent manner resulting in tumors with high antigen load exhibiting early CAR T cell accumulation whereas in antigen low lesions the immune cell accumulation proceeds relatively slow 34 .…”

“…Although this observation is in line with several other studies 34 , 58 , 59 , Sellmyer and colleagues reported discrepant findings in which both therapeutic and control T cells initially homed to the spleen followed by an accumulation at the tumor site 27 . This contradiction can be explained by the study of Skovgard and colleagues who demonstrated that CAR T cell accumulation occurs in an antigen-dependent manner resulting in tumors with high antigen load exhibiting early CAR T cell accumulation whereas in antigen low lesions the immune cell accumulation proceeds relatively slow 34 . In fact, in our model, EGFR is an abundantly expressed tumor antigen ( Figure S8 ), whilst Sellmyer and colleagues used GD2 for targeting which is expressed heterogeneously and at relatively low levels in tumor cells 60 .…”

“…Ideally, these methodologies should enable longitudinal analyses as - owing to their nature as living drugs - CAR T cells actively traffic throughout the body and undergo dynamic expansion and contraction kinetics which cannot be comprehended by end-point analyses. Thus, current efforts focus on imaging-based approaches applying BLI and/or PET to study CAR T cell characteristics on a whole-body scale with temporal reconstruction 26 - 28 , 30 - 32 , 34 , 35 . In this context, Sellmyer and colleagues proved the combination of BLI with PET particularly advantageous as it harnesses the high detection sensitivity of BLI with an improved spatial resolution of PET/CT 27 .…”

“…Thus, optical imaging is establishing itself as a facile and inexpensive alternative that allows the analysis of multiple animals simultaneously. In this regard, optical luciferase-based reporter gene systems were successfully applied to characterize CAR T cell responses even at low T cell numbers and for a prolonged period of time 34 , 35 . Yet, a drawback of these studies was poor spatial resolution that was restricted to the 2-dimensional (2D) space and complicated precise delineation of the therapeutic cells' localization.…”