In a clinical trial evaluating a novel immunotherapy option for cancer treatment, a child with rhabdomyosarcoma, a form of muscle cancer, that had spread to the bone marrow, showed no detectable cancer following treatment with chimeric antigen receptor (CAR) T cells that were engineered to target the HER2 protein on the surface of the cancer cells.
The trial, conducted by researchers at Baylor College of Medicine, Texas Children’s Hospital and Houston Methodist Hospital, was recently published in the journal Nature Communications.
“This child’s cancer was considered high risk because it had not responded to standard chemotherapy. As a result, this child was a candidate to receive a promising new CAR T cell therapy, a personalized form of immunotherapy that redirects the patient’s own immune T cells to recognize and fight the tumor,” said first and corresponding author Dr. Meenakshi Hegde, assistant professor of pediatric hematology-oncology at Baylor College of Medicine and Texas Children’s.
About 75 percent of the tumor cells in this patient displayed a protein on their surface called HER2. The researchers reprogrammed the T cells to target the HER2 protein by genetically engineering them to express CAR molecules that recognize the HER2-expressing (HER2+) cancer cells.
In a previous clinical trial, the HEROS study, the researchers found that CAR T cells directed at HER2+ tumor cells had a favorable safety profile.
This early generation CAR T cell treatment resulted in clinical benefit in a small subset of patients, but it did not eradicate their tumors.
“From the HEROS trial, we learned that HER2-CAR T cells expanded but did not persist in the patients, which could in part explain the lack of anti-tumor responses,” said Hegde, who also is part of Baylor’s Dan L Duncan Comprehensive Cancer Center.
To overcome this limitation, Hegde and her colleagues added successive HER2-CAR T cell infusions along with low-dose chemotherapy to delete normal T cells as a strategy to improve the expansion and persistence of the infused HER2-CAR T cells in a trial they called, HEROS 2.0.
The lymphodepleting chemotherapy administered before transferring HER2-CAR T cells eliminated the patient’s existing immune cells, creating a space for the engineered CAR T cells to expand in the patient.
“Although the child had a lasting response to HER2-CAR T cells with no tumor detected, the cancer returned six months after we stopped the T cell infusions.
Fortunately, the child achieved a second remission after retreatment with HER2-CAR T cells,” Hegde said. “Considering the several challenges in successfully treating solid tumors using CAR T cells, achieving this exceptional tumor response is very encouraging.”
At the time of this report, the child is 19 months off T cell treatment and remains healthy and cancer free.
New insights into how this exceptional recovery occurred
The sustained tumor response in this child has provided the researchers important insights into how the cancer was eliminated. The CAR T cells were developed to recognize and attack HER2+ cancer cells. Although not all cancer cells expressed HER2 on the cell surface, the tumor was eliminated in its entirety prompting the question of how the HER2-negative cancer cells were eradicated.
“We found evidence suggesting that, following the infusion of HER2-specific CAR T cells, the patient’s own immune system was recruited to act against the tumor, which might help explain the durable complete response,” Hegde said.
“We plan on conducting more detailed experiments in a larger group of patients treated with HER2 CAR T cells to better understand the involvement of the patient’s immune system in eliminating the cancer.”
“It is fascinating to see remodeling of the patient’s T cell compartment and development of antibodies directed against proteins implicated in tumor survival and metastasis during the course of treatment in this child.
The immune activation mechanisms and associated tumor targets unfolded during the acquired response, could inform novel approaches to fight difficult-to-treat cancers,” said Dr. Sujith Joseph, senior scientist at Baylor’s Center for Cell and Gene Therapy, who conducted the in depth evaluation of the patient’s immune response.
“This study shows that CAR T cells could perhaps act as vaccines by exposing cancer proteins to the patient’s immune system. With more understanding and further refinement of their design, CAR T cells could be effective against some incurable malignancies,” said senior author Dr. Nabil Ahmed, associate professor of pediatrics and immunology at Baylor and Texas Children’s Hospital.
Understanding of the intricate relationship between the immune system and the tumor cells has provided the accelerated development of cancer treatment such as rejuvenation of host immunity, training the immune cells against the cancer cells, removing the exhaustions of immune cells against tumor antigens etc.
Turning on immune cell against the cancer cell either through antibodies mediated therapeutics or re-infusion of trained immune cells into the cancer patient are excellent examples of cancer immunotherapy [1, 2, 3].
As tumor cells utilize various escape mechanisms for their survival from immune surveillance, the vital question is how the immune cells can be genetically reprogrammed to recognise and kill tumor cells.
Extensive research employing immune cells in cancer treatment has led to the development of antibodies based immunotherapies , allogeneic stem cell transplantation  and Chimeric Antigen Receptors (CARs) T cell therapy .
Among all the cell-based therapies, the genetically engineered CAR bearing T-cells (CAR T cell) for targeting cancer cells has emerged as the successful therapy in hematological malignancies.
Its unprecedented role in cancer treatment [6, 7, 8, 9] has been due to better understanding of transcriptomics, proteomics, genomics, and Cell Based Assays. In 2017, two anti-CD19 CAR T-cell therapies have been approved by the US Food and Drug Administration (FDA) Tisagenlecleucel (Kymriah, Novartis) and Axicabtageneciloleucel (Yescarta, Kite Pharma).
However, the long term follow ups have shown [10, 11] the development of resistance against CAR T cell therapy, which needs further evaluation . Plenty of opportunities to improve the efficacy and safety of this therapy may establish a paradigm of CAR T cell therapy not only in haematological malignancies but also in solid tumors which frequently exhibit resistance to it.
Hence, we have discussed the main keys of CAR T cells focussing on ‘how to improvise it to minimize failures and relapses’ for a better outcome. This includes the discussion on the effect of tumor mutation burden (TMB), tumor microenvironment (TME), role of epigenetic mechanisms and possible combination therapies.
Until the CAR T cell therapy is made affordable, its potential to reduce the disease burden will remain untapped. Hence, manufacturing cost reduction should be prioritized by assessing the need (therefore the demand), availability of skilled staff and infrastructure, which may contribute towards cost minimization and thus a wider use. Recent observations regarding the cancer cases, which are increasing remarkably every year, are quite alarming (Figure 1).
Thus, the need of establishing better therapies at an affordable cost is our priority. We propose a triangular collaboration among hospital, academia and industry which may further improve the quality and affordability of this cellular therapy, especially for economically developing nations.
Strategies in CAR T cell therapy
Successful CAR T cell therapy has been developed by careful considerations of crucial factors such as
- Selection of cancers where it can act optimally;
- Identification of appropriate tumor antigen(s);
- Selection of gene constructs to design chimeric receptors;
- Selection of immune cells for engineering the receptors against tumor antigen;
- Selection of other determinants of efficacy against cancer cells such as affinity of CAR for tumor antigen(s);
- Selection of appropriate growth factors in the development of CAR T cells;
- and potential toxicities associated with the therapy including cytokine release syndrome and neurotoxicity.
With the advent of new technology such as real time cellular assay and next generation sequencing, these crucial factors may be exploited to further improve CAR T cells’ performance (Figure 2).
The CAR T cell therapy is based on the nature of cancer cells (suppressive or infiltrative) carrying specific tumor antigen(s) . Hematological malignancies have been treated successfully by CAR T cell therapy as compared to solid tumors .
For instance, CD19-antigen selection in hematologic malignancies was near to perfection due to the readily available antigen for its recognition by anti-CD19 CAR T cells.
However, in the case of solid tumors, CAR T cells generally failed to interact with the tumor antigens due to physical barriers and an immune suppressive tumor microenvironment (TME) . Still successful approaches for solid tumors have been proposed and reported .
So far, the most successful antigen has been CD19, a tumor associated antigen (TAA) and not the tumor specific antigens or Cancer-germline/cancer testis antigens . Though, other TAAs (CD20, CD22 or CD19 and CD22 together, CD 30) have also been used in various cancer immunotherapies [16, 17, 18, 19]. Various other tumor antigens may also give directions and opportunities to design receptors appropriately .
The effective cytotoxic T cell in CAR T cell carries a genetically engineered receptor protein on its surface, equivalent to T Cell Receptor (TCR) but recognises tumor antigen independent of Major Histocompatibility Complex which is an added feature.
Many types of CARs have been designed and successfully used . To reduce the cost of CAR T cell therapy, the concept of Universal CAR (UniCAR) and universal T cell has emerged .
The UniCAR contains a bridging molecule which serves as a connector through a domain directed against a tumor antigen and an epitope being recognized by the signalling domain of a conventional CAR. Thus, T cells with UniCARs may be prepared from allogenic healthy donors avoiding the patient’s blood which may have low T cell count.
The graft-versus-host disease, which is adverse immunologic response due to transplant, may be abolished by genetic elimination or disruption of the TCR gene and/or HLA class I loci of the allogenic T cells . However, this needs further validation before being practised. This approach may be very useful in cost reduction of the CAR T cells as it will abolish the initial steps involving patient.
Earlier, a similar technique where immune cells isolated from the tumor region, grown outside of the body and re-infuse back to the patient to kill cancer cells, was described as Adoptive cell transfer (ACT) .
The choice of immune cells for ACT is a critical step as there are several of them, namely, Natural Killer cells, Tumor-infiltrating lymphocytes, T cells and dendritic cells . Hence, any of these cells can be genetically modified to reprogram them against tumor cells.
However, T cells have successfully been used in most cases for developing cell based immunotherapies  owing to their intrinsic properties of proliferation, cytotoxicity and memory. The efficacy and persistence of CAR T cell in-vivo decides the successful outcome of the therapy.
Therefore, the factors contributing towards their effector functions are taken into consideration in the existing approaches. The cellular components (other T cell subtypes) the use of growth factors and interleukins for CAR T cells’ activation and proliferation have been found to affect the performance of CAR T cells in-vivo [23, 24, 25].
Therefore, leukemic cells must be depleted before isolating T cells for CAR T cell preparation [9, 24]. Equally important is the ratio of CD4+ to CD8+ or total T-cell isolated from the patients [17, 26].
Some studies have reported that it could be difficult to isolate sufficient number of T cells from patients with relapsed/refractory cases or those that had multiple rounds of chemotherapy.
Also, due to heterogeneity among the patient’s blood samples, the proliferation and efficacy of CAR T cells prepared, have shown different functional ability, although sufficient quantity of CD3+ lymphocytes were isolated to manufacture CAR T cells .
In summary, it is essential to better understand the different strategies of CAR T cell therapy (summarised in Figure 2) for the development of newer approaches for cancer treatment.
Failures and relapses in most cancer treatments have been reported and CAR T cell therapy is no exception as individual immunity and co-morbid conditions vary among cohorts . Understanding these events is the next milestone for better results of this therapy.
Long term survival studies in CAR T cell therapy have indicated cases of disease relapse within one year of treatment [10, 11]. In a rare case, one patient who initially did not respond to therapy showed complete remission after clonal evolution of one of the CAR T cell clones with hypomorphic mutation in one of its tumor suppressor genes .
On the contrary, a relapsed case was reported in a B cell acute lymphoblastic leukemia with aberrant myeloperoxidase expression after CAR T cell therapy . These findings suggest the importance of mechanistic studies on CAR T cell therapy with more cases to understand the altered gene expression exhibiting two opposite phenomenon- one remission and the other, relapse after the therapy.
To get a complete picture of the events occurring in failure and relapses, the strategies employed by the cancer cells to escape CAR T cell need special attention [31, 32].
In general, tumor cells escape by –
- Lineage switching [33, 34];
- loss of tumor antigen, for example CD 19, or epitope hiding from recognition ;
- Immunomodulation of the host immune cells to escape from surveillances ;
- T cell exhaustion and epigenomic landscape modulation .
Examples, such as lineage markers including myeloid conversion in patients following CD19 CAR therapy is seen in murine adult acute lymphoblastic leukemia (ALL) models after the long-term effects of CD19 CAR-T cells . Also, a CD19-negative myeloid phenotype is responsible for the immune escape of mixed-lineage leukemia (MLL) from CD19 CAR-T-cell therapy .
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More information: Meenakshi Hegde et al, Tumor response and endogenous immune reactivity after administration of HER2 CAR T cells in a child with metastatic rhabdomyosarcoma, Nature Communications (2020). DOI: 10.1038/s41467-020-17175-8