Take Home Messages
  • The use of CAR T-cell therapies has grown significantly over the past decade. The future of CAR T is in identifying which patients benefit, enhancing the efficacy of CAR T-cell therapy, and expanding access to treatment.

  • “On-target, off-tumor” effects are a major barrier to developing solid tumor CAR T-cells, because most potential CAR T targets are also expressed on healthy tissue.

  • “Logic gating” is a strategy that combines responses to multiple tumor surface antigens to make CAR T-cells more sensitive and specific for malignant tissue.

  • CAR T-cells for solid tumors must persist and proliferate in the tumor microenvironment. “Armored” and tissue-targeted CAR T-cells are among many mechanisms for improving the efficacy and specificity of solid tumor CAR T-cells.

Cell Therapy Overview- Recent Advances and Future Directions

Since Bill Ludwig was first treated on a clinical trial in 2010, at least 10,000 patients have been treated with CAR T-cells.1 As CAR T continues its second decade, the next steps for moving the field forward are to improve the efficacy of existing CAR T, expand the range of diseases that can be treated with CAR T, and combat the unique toxicities of CAR T therapy.

Diffuse large B-cell lymphoma (DLBCL) is an example of where CAR T has both rapidly shifted treatment paradigms and where it still has room to optimize who is treated and when. Second line CAR T-cell therapy for DLBCL has made major strides as the standard of care, but the long term five year results of ZUMA-1 demonstrate a five year overall survival of only 43% and disease free survival of only 31%.2 While many patients who responded appear to have reached a durable cure, more work is needed to define who will truly benefit from CAR T and to expand the percentage of those patients who can access CAR T therapy. For those patients responding to salvage therapy, it remains unclear whether CAR T or autologous stem cell transplant provides a greater chance of cure, as the poorer performance of the salvage arm in ZUMA-7 was largely driven by poor response to salvage chemotherapy rather than progression after transplant.3Additionally, whether high-risk patients with DLBCL would benefit from first-line CAR T is currently under investigation with ZUMA-23.4 These examples demonstrate the need to identify which subpopulations of patients receive the maximum benefit from CAR T in order to optimize efficacy and conserve limited apheresis opportunities.

Enhancing the efficacy of the CAR T product itself is key to improving outcomes. Relapse after CAR T may be due to either lack of persistence of the CAR T product itself, or resistance via antigen escape. In broad terms, CAR T-cell fitness can be improved by avoiding T-cell toxic agents such as bendamustine prior to apheresis,5 and strategies are under investigation to enhance CAR T-cells themselves with “armor” to prevent fatigue and immune downregulation,6,7 or use allogeneic products from healthy donors which may be less prone to fatigue. Multiply-targeted products may also be useful to combat antigenic escape.

In addition to enhancing efficacy, improving outcomes requires understanding the toxicities of CAR T therapy. These include both generalized “off-target” toxicities and “on-target off-tumor” toxicities. Generalized toxicities include cytokine release syndrome (CRS), immune effector cell associated neurotoxicity syndrome (ICANS), long-term cytopenias, immune effector cell-associated hemophagocytic syndrome (IEC-HS), and movement and cognitive disorders in BCMA therapies. Development of better T-cell therapies must consider both maximizing efficacy and reducing toxicity.

Lastly, improving access to CAR T therapy and reducing racial disparities in access are tantamount to improving patients’ lives with this new technology. At each step of the process to cell infusion—diagnosis, referral for cell therapy, collection, manufacturing, and infusion— patients who may benefit fall out of the pipeline to treatment. To improve this, stakeholders will need to work toward streamlining and scaling manufacturing, educating patients and referring physicians, reducing the overall cost of CAR T therapy, and measuring and advocating for the value of CAR T with insurance and policymakers.8

Figure 1
Figure 1.Improving T-cell therapy access. Patients may be lost at several steps in the CAR T therapeutic pathway. A multifocal approach is necessary to enhance access to cellular therapies. (adapted from Komanduri, JCO 2021)8

Actionable insights

  • To maximize the impact of CAR T therapy, we must optimize identification of patients who will benefit from CAR T and enhance the biological environment that the CAR T-cell encounters.

  • Access to CAR T therapy is a major issue, and clinicians should ensure that they are referring patients equitably into treatment.

Novel signaling strategies for CAR-T therapies in Solid Tumors

A critical barrier to developing CAR T-cells for solid tumors is developing cells which can distinguish normal tissues from cancer.9 CAR T-cells are engineered to target specific antigens expressed on tumor surfaces; when these antigens are also expressed on healthy tissue, the cells can cause severe “on-target, off tumor” toxicity. This toxicity limits the implementation of solid tumor CAR T-cells because almost all potential CAR T-cell targets are also expressed on healthy tissue.10 In light of these limitations and ongoing efforts to enhance CAR T potency, which could worsen “off-tumor” toxicity, two strategies are under development to widen the therapeutic window for CAR cells in solid tumors: logic gating and targeting tumor specific antigens.

“Logic gates” are an engineering solution to improve cells’ sensitivity and specificity to tumor, rather than healthy tissue. The simplest type is an “AND” gate: an engineered element in the CAR T-cell which requires display of two antigens, rather than one, to activate the T-cell.11 For example, lung adenocarcinomas are more likely than benign lung tissue to express both CA9 and TREM1; a CAR T-cell which requires activation by both might have a stronger “on-tumor” effect with less “off-tumor” damage.12 similarly, a NOT gate may distinguish situations in which tumor cells have loss of surface expression of cell markers which are seen on healthy tissues. For example, many CEA positive tumors, such as pancreatic cancer, have lost HLA-A*02 expression. A CAR T-cell with a “NOT” logic gate to activate against cells with CEA expression but not HLA-A*02 expression would be more specific for malignant, rather than benign cells. These basic logic gates can be combined and many such concepts are under testing now.13 Additionally, an emerging next step in logic gating is to respond to the tumor microenvironment, such that T-cells are only primed for killing when exposed to the unique tissue environment of a tumor and inactive while in healthy tissue.14

A second strategy is developing CAR T-cells targeted to tumor-specific antigens. Today the majority of developed CAR T-cells have targeted tumor-associated antigens, which are self-antigens expressed by tumor cells which may be enriched or overexpressed in tumor cells but can be found on some normal host cells as well. Novel CAR T-cells are under development to target tumor-specific antigens, which are neoantigens or non-self-antigens expressed by tumor cells. These are not present in normal cells and arise either as neo-antigens or as re-expression of onco-fetal antigens. Because they are not expressed on healthy cells’ surfaces, they may reduce the “on-target, off-tumor” effect of traditional tumor-associated antigen CAR T-cells.12

Major challenges to implementing these “logic gated” or tumor-specific antigen-targeted solid tumor CAR T-cells are accounting for tumor heterogeneity and plasticity as well as overlapping expression on normal tissues. However, next generation CAR T products are already in development to bring these enhanced strategies to a broad range of solid tumors in the coming years, and as CAR T engineering improves, the therapeutic window for cellular therapy will widen to include solid tumors.

Actionable insights

  • Solid tumor CAR T-cells are limited by “on-target, off-tumor” toxicity. Several strategies are in development to mitigate this via increasing CAR T-cells’ specificity towards malignant cells.

Improving the Success of CAR T- Cells and Applications to Solid Tumors

In addition to these strategies for enhancing the safety and tolerability of CAR T-cells for solid tumors, CAR T-cells will need stronger and more persistent effect to treat solid tumors. Areas of progress can be divided into four categories: enhancing CAR T-cell trafficking and infiltration, addressing tumor heterogeneity and antigen escape, promoting proliferation and persistence, and combating an immunosuppressive tumor microenvironment.

Solid tumor CAR T-cells have a relatively unique need to anatomically target non-vascular or lymphatic sites, as compared to CAR T-cells for hematologic malignancies. Promoting trafficking and infiltration to solid tumors may improve both efficacy in attacking tumors and safety in avoiding healthy tissues. To promote trafficking into tumors, CAR T-cells with chemokine receptors and CAR T-cells which target antigens associated with tumor stroma or vasculature (such as VEGFR2 or FAP) are under development.15 Once CAR T-cells have arrived to a tumor tissue, heparanases have been added to CAR T-cells to help them break down tumor stroma and reach malignant cells.16 To keep CAR T-cells in the tissues, inhibition of cell trafficking molecules has been proposed to prevent CAR T-cells from exiting the tumor environment. Lastly, for some tumor sites, such as the CNS or gut, intra-lesional or intracavitary CAR T-cell injection has been proposed to spatially target cells directly to solid tumors.17

Once present, CAR T-cells have to be kept active and resist an immunosuppressive tumor microenvironment. Strategies under development to enable cells to proliferate and persist include costimulatory domains, dual targeting, knocking in or editing genes to counter exhaustion, or altering surface cell signaling molecules associated with fatigue.18,19 “Armored CARs” which release their own inflammatory cytokines or CAR T-cells which have undergone gene editing to ignore immunosuppressive molecules and hostile tumor environments may also enable cells to persist and prevail.20

While there is a theoretical risk of transformation events, in which CAR T-cells become malignant, this has not been observed frequently in humans to date. Recently, the United States Food and Drug Administration has announced it is examining rare T cell malignancies in recipients of CD19- and BCMA-directed CAR-T products; as of this writing causality has not been determined (e.g., by examining vector integration and clonality studies) and the overall incidence of secondary lymphoma, relative to the number of recipients to date, appears to be very low (perhaps ~0.1%). As new technologies make CAR T-cells more persistent and proliferative, safety mechanisms such as “suicide genes” or elimination genes may allow for targeting of CAR T-cells if malignant transformation occurs.21

Many strategies are in development for strengthening and prolonging the effect of CAR T-cells in solid tumors. Next-generation cellular products will combine these techniques and novel means of targeting tumor cells, making solid tumors the next great frontier for cell therapies in cancer.

Actionable insights:

  • Solid tumor CAR T-cells may benefit from tissue-directed delivery or trafficking and from design features which make them hardier to the tumor microenvironment.

CAR-T therapy for hematologic malignancies: recent advances and future directions

CAR T-cells in hematologic malignancies represent the major breakthrough of cellular therapy to date. Currently 4 CAR T-cell products are approved for B-cell malignancies, each with distinct patterns of CAR-T expansion and persistence: the CD28 CARs axi-cel and brexu-cel have more rapid expansion, shorter persistence, and a greater rate of CRS/ICANS, while the 4-1BB CARs tisa-cel and liso-cel have slower expansion, longer persistence, and less CRS/ICANS. As described above, these therapies have an approximately 30-40% rate of cure in refractory DLBCL, with strategies under way to improve the number of patients who benefit.2,22 Similarly, in relapsed/refractory multiple myeloma, cilta-cel and ide-cel are approved after four or more lines of prior therapies and FDA approval has been submitted to move CAR T-cells to earlier lines of therapy after recent improvement in progression-free survival versus standard of care in the second and third line settings.23,24

As described above, major mechanisms of resistance to CAR T-cells in hematologic malignancy are T-cell exhaustion, antigen escape, and the tumor microenvironment. “Armored CAR T-cells” are in development in the hematologic malignancy space as well to engineer CAR T-cells to secrete cytokines to enhance CAR T proliferation and the tumor microenvironment.6 Additionally, to enhance access to cellular therapies, allogeneic, or “off-the-shelf” CAR T-cell and CAR NK-cell products are under development, though these are limited by the risk of graft-versus-host disease (GVHD), host rejection, and the risks of intensified lymphodepletion needed prior to infusion. Genome editing has been proposed to minimize the risk of GVHD.25 All of these strategies may enhance the availability, efficacy, and tolerability of CAR T-cells in hematologic malignancies.

Actionable insights:

  • CAR T-cells are now well-established as standard of care therapies in hematologic malignancies. Cellular therapies may move into earlier lines of therapy and allogeneic CAR therapies may help improve access.

Conflict of Interest

  • Krishna Komanduri: Ad hoc Consulting: Iovance, Incyte, BMS, Cargo Therapeutics,Instil Bio, CRISPR therapeutics, Genentech/Roche. Scientific Advisor: Aegle Therapeutics, Avacta Therapeutics

  • Michael Spinner: Consulting for Gilead/Kite

Funding Information


Ethical Statements



These proceedings cover the presentation of Dr. Julia Carnevale from the University of California San Francisco, whose presentation is reviewed in the section “Improving the Success of CAR T-Cells and Applications to Solid Tumors”. The authors thank the Binaytara Foundation for the opportunity to highlight this important topic.

Author Contributions

  • All authors: conception and design

  • All authors: data collection and assembly

  • HA and IA: data analysis, manuscript writing

All authors have approved this manuscript