Limitations of CAR-T
1. Cytokine storm
This is one of the main adverse reactions of CAR-T technology in clinical application. The release of cytokines caused by the large increase of T cells causes the body to develop symptoms such as fever, myalgia, hypotension, and respiratory failure. In response to the cytokine storm caused by CAR-T cell reinfusion, tocilizumab, an IL-6 receptor antagonist, can be used clinically to alleviate those symptoms. At the same time, a research team also observed that the cytokine storm was also related to the degree of disease progression or tumor burden. In patients with high disease burden, there was a higher release of cytokines.
2. Targeted cytotoxicity
Because the CAR-T antigen is highly targeted, it is impossible to distinguish tumor cells and normal cells expressing the corresponding antigens, so both are aggressive. For example, CAR-T cell therapy for CD19 leads to dysplasia of B cells, and CAR-T cell therapy targeted to Her-2 can cause attacks to other tissues or organs outside the tumor site, such as cardiopulmonary system toxicity. Therefore, choosing the right CAR-T for tumor-specific antigen synthesis, colleagues who are attacking tumor cells can distinguish normal cells from being attacked.
3. Nervous system toxicity
During the treatment of leukemia with CAR-T, several research groups observed neurological adverse effects such as mental disorders and speech disorders in a small number of patients.
4. Other cytotoxicity
It might cause autoimmune disease. Therefore, the development of new effector cells with strong antitumor effects has important theoretical significance and clinical application value.
Advantages of NK cells
NK cells are considered as effector cells that have the potential to enhance their anti-tumor capabilities through CAR modification because of their special recognition mechanism of target cells, short physiological cycles, and extensive tumor-killing ability. NK cells are a type of MHC-independent lymphocytes that have a strong killing effect on tumor cells. Their recognition of tumor cells mainly depends on the cross-regulation of their surface-activating receptors and inhibitory receptors. After identifying tumor cells, NK cells kill tumor cells by releasing killing mediators of perforin and granzymes to target cell apoptosis, expressing membrane TNF family molecules to induce target cell apoptosis, and antibody-dependent cytotoxicity. And NK cells do not need match to specific patients.
The main advantages of CAR-NK
1. No need to activate endogenous cytotoxic receptors of NK cells
2. Selective identification of tumor-associated antigens
3. Organic antigen-specific target cell lysis
4. It has obvious advantages for the treatment of solid tumors. Solid tumors show different degrees of tolerance to unmodified NK cells, but they are sensitive to antigen-dependent NK cells.
The ideal effector cells for CAR modification generally have the following characteristics:
1. Amplification can be achieved in vivo or in vitro to meet the requirements of adoptive infusion;
2. Sufficient tumor killing ability;
3. Able to reach the tumor area;
4. No side effects.
The population of NK cells is heterogeneous, and different populations have their own advantages and disadvantages. Choosing one of the appropriate effector cells can not only maximize the potential of CAR modification, but also help control the cost of treatment and increase the possibility of clinical transformation.
Source of CAR-NK
There are three main sources of CAR-NK cells: peripheral blood, NK cell lines, and stem cells with differentiation potential.
The main source is peripheral blood. In the past decade, there have been more effective methods to purify and expand a sufficient amount of NK cells from peripheral blood and peripheral blood stem cells for immunotherapy, and gradually used in the development of CAR modifications. Both autologous and allogeneic NK cells can be CAR modified, but the cell characteristics are different. Allogeneic NK cell donors have other lymphocytes, mainly T cells, in the blood. The presence of these heterocells can cause plant-versus-host disease (GVHD). Therefore, T cells must be removed before being used for treatment. Allogeneic NK cells do not match the HLA-I molecules of the patient because the surface killer immunoglobulin-like receptors do not match the patient's HLA-I molecules, so they have sufficient killing activity against CAR-targeted tumor cells. After autologous CAR-NK cells are reinfused, their inhibitory receptors bind to HLA-I molecules expressed by autologous normal cells to generate an inhibitory signal, which will inhibit the killing effect of NK cells. Although classical HLA-I molecules are lost from tumor cells, the expression of non-classical HLA-I molecules (HLA-G, HLA-E, etc.) can also inhibit the activation of NK cells.
In addition to primary cells, NK cells have many mature cell lines, including NK-92, NKG, NK-YS, and NKL, among which NK-92 is the most widely studied. NK-92 is derived from the peripheral blood of a woman with non-Hodgkin's lymphoma. It was established in 1992 and is an IL-2 dependent immortal cell line. It has strong cytotoxicity and high expression of a series of molecules related to cytolysis. NK-92 expresses a part of activated receptors on its surface, such as NKG2D and tumor cells expressing NKG2D ligands such as acute lymphoblastic leukemia cells, which are extremely sensitive to the killing effect of NK-92. Compared with primary NK cells, the biggest advantage of NK-92 cells is that the expression of inhibitory receptors on their surface is very low. The lack of inhibitory receptor signals makes them more effective in killing various tumors than primary NK cells or other killer cells that undergo cytokine sparks.
In addition to NK cell lines and NK cells derived from peripheral blood, NK cells derived from induced pluripotent stem cells, umbilical cord blood, and embryonic stem cells also have the potential for CAR modification. The phenotype of NK cells differentiated from these stem cells is similar to that of NK cells derived from peripheral blood, and their growth ability is stronger, which can meet the requirements of clinical applications.
To be continued in Part II…