Chimeric Antigen Receptor Transgenic, T Cell Receptor/CD3 Negative Monospecific T Cells Generated from Cord Blood CD34 Positive Cells

Autologous T cells transduced to express chimeric antigen receptors (CAR) directed against CD19, a B cell antigen, are reported to induce complete remission in patients with leukemia or lymphoma of the B cell lineage. Although potentially very effective, this treatment strategy has major drawbacks. First, CAR therapy is based on autologous T cells and therefore dependent on the nature and quality of T cells present in the peripheral blood of these patients at the time of treatment. Poor quality of the T cells may cause treatment failure in some patients. In addition, therapy based on autologous cells is tailor-made i.e. CAR⁺ T cells have to be generated de novo for every patient. Finally, autologous cell therapy requires different, more complicated logistics than conventional therapy. We therefore investigate whether a general purpose, allogeneic CAR therapy based on HLA-matched cord blood obtained from cord blood banks can be devised. Here, we investigated whether functional CAR+ T cells can be generated in vitro that do not express an endogenous T cell receptor to avoid alloreactivity causing graft versus host reactions.

We compared carcino-embryonic antigen (CEA)- specific CARs of the first generation (intracellular CD3ζ signaling chain), of the 2^nd generation (intracellular CD3ζ and CD28 signaling chain) and of the 3^rd generation (intracellular CD3ζ, CD28 and OX40 signaling chain). CD34⁺ progenitor cells were isolated from human cord blood or postnatal thymus and subsequently transduced with one of the three green fluorescent protein (GFP)-encoding CAR constructs. Transduced cells were subsequently co-cultured on OP9DL1 in the presence of stem cell factor, Flt3-ligand and interleukin-7. Unlike T cell receptor transduced precursors (1), expansion was not enhanced by transduction of the chimeric receptor. Expansion was highest with first generation CARs whereas second and third generation CARs displayed only restricted expansion. Similar to T cell receptor transduced progenitors, CAR transduced cells show an accelerated differentiation during co-culture compared to the non-transduced cells: first committed CD5⁺ CD7⁺ T precursors appear, then CD4⁺ CD8⁺ double positive cells (DP) and finally CD1^- CD27^- single positive or double negative (DN) mature T cells. In cultures transduced with 2^nd and 3^rd generation CARs, few transduced cells passed through the proliferative DP pathway but rather differentiated to mature CD1^- CD27^- non-proliferative DN cells without passing through the DP stage. This phenomenon is responsible for the limited expansion seen with precursors transgenic for 2^nd or 3^rd generation CARs. However, in all cultures CAR⁺ DP cells were generated and, as shown for TCR transgenic cells (1), we were able to induce CEA specific maturation after co-culturing these DP cells with a cell line expressing CEA or by antibody-induced cross-linking of the CAR, giving rise to CD1^- CD27⁺ matured cells. The observations described above are compatible with data obtained in mice showing that strong T cell receptor (TCR) activation during thymocyte differentiation inhibits the generation of DP cells and induces maturation to DN cells. Both the spontaneously and induced mature CAR⁺ cells were TCR and CD3 negative, suggesting that the expression of a CAR in early T cell precursors shuts down rearrangements of the endogenous TCR chains. Moreover, these cells lack NK marker expression (CD56, NKG2D) and show expression of T cell markers (CD5, CD7, CD2), confirming their T cell nature. In conclusion, the CAR⁺ CD3/TCR negative cells are T cells as these are derived from T cell precursors (CD5⁺, DP cells) and express various membrane and nuclear T cell markers.

Mature CD1^- CD27^- CAR⁺ cells can be expanded to large cell numbers using T cell expansion protocols. They displayed cytokine production specific for CEA⁺ tumor lines as well as specific cytotoxicity. Moreover, the 2^nd and 3^rd generation CAR expressing cells showed increased specific cytokine production when compared to the first generation CAR expressing cells. These results show that the cord blood-derived CAR⁺ cells have potent functional activity similar to peripheral blood derived CAR⁺ T cells.

We believe that these in vitro generated CAR⁺ cells developed from HLA-matched cord blood progenitors may be ideal as an adjunct to cord blood transplantation.

(1) Snauwaert et al, Leukemia, 2014