Financial barriers, technical challenges (e.g., inadequate leukapheresis or rapid disease progression), and disease-related obstacles (e.g., T-cell dysfunction in certain malignancies) often limit the applicability of CAR-T-cell therapies, and there has thus been significant interest in the development of allogeneic, or so-called “universal,” CAR-T products. To accomplish this, several groups have reported the use of gene editing techniques such as CRISPR-Cas9, transcription activator-like effector nucleases, and zinc-finger nucleases to knock out the endogenous αβ-TCR in donor T cells, thereby mitigating the risk of GVHD associated with the infusion of donor-derived CAR-T cells. Several universal CAR-T products are currently being evaluated in clinical trials, although it remains to be seen how such approaches will affect CAR-T-cell function, persistence, and associated toxicities.
Additional strategies for the development of allogenic, “off-the shelf,” cellular therapies include the use of alternative immune effectors that do not mediate significant alloreactivity, such as gamma delta (γδ) T cells, NK cells, NKT cells, and VSTs. NK cells are effector cells of the innate immune system and are known to exhibit significant anti-tumor activity via multiple cytotoxic mechanisms despite lacking an antigen-specific receptor analogous to the TCR. CAR-NK cells have several potential advantages over CAR-T cells; the possibility of both CAR-dependent and CAR-independent mechanisms for activation; a decreased risk of on-target, off-tumor toxicity; a decreased risk of GVHD; and a lower risk of cytokine-mediated toxicities. Several early-phase clinical trials are currently underway, including trials evaluating CAR-NK cells targeting CD19, BCMA, CD33, and CD7.
NKT cells express NK lineage markers in addition to a highly invariant αβTCR (in contrast to the highly diverse, antigen-specific TCRs of T cells). The invariant αβTCRs of NKT cells recognize lipid antigens presented by the MHC class-1-like molecule, CD1d. CAR NKT cells can exhibit antitumor activity via CAR-mediated activation, but also via cytokine secretion leading to activation of T and NK cells. Similar to NK cells, allogeneic donor-derived NKT cells have a lower risk of alloreactivity compared with T cells. The feasibility of CAR-NKT has been demonstrated in several pre-clinical studies, and initial clinical trials are currently underway.
Gamma-delta T cells represent another potentially useful T-cell subset for use in both autologous and allogeneic CAR-T-cell therapy. γδ T cells represent a small peripheral blood T-cell subset with features of both the innate and adaptive immune system, expressing NK cell receptors that target stress-inducible surface molecules, while also expressing clonally rearranged γδ TCRs that recognize antigens in an MHC/HLA-independent fashion. The HLA-independence of γδ TCRs likely explains the lower potential for alloreactivity. Gene modified γδ T cells have been evaluated in multiple pre-clinical studies, though this approach is still early in development.
Adoptive cellular therapy with VSTs has been used for many years to target CMV, EBV, and other viruses common after allogeneic HSCT. VSTs were initially derived from allogeneic stem cell donors, but patients have also been successfully treated with banked VSTs derived from partially HLA-matched healthy donors. Because the latter approach is associated with a low incidence of GVHD, allogeneic CAR-VSTs have been identified as another potentially useful “off the-shelf” cellular therapy strategy with a low risk of complications related to alloreactivity.
