CD28 Costimulation of Effector T Cells During An Anti-Tumor Attack Sustains Its Repression by Regulatory T Cells Which Can Be Overcome by Preventing Lck Activation.

Massive infiltrations of the tumor tissue with CD4⁺CD25^highFoxP3⁺ regulatory T (Treg) cells render a productive T cell anti-tumor response ineffective resulting in unrestricted tumor growth despite the presence of tumor-specific, cytolytic T cells.

Using a human/CD1-deficient mouse model we investigated the impact of human regulatory T cells on redirected effector T cells expressing a tumor-specific chimeric antigen receptor. The chimeric antigen receptor consists of an antibody derived binding domain for antigens in the extracellular part and of the TCR/CD3zeta or the combined CD3zeta-CD28 signaling domain in the intracellular part. Upon antigen binding the chimeric antigen receptor transmits an activation signal via the CD3zeta or CD3zeta-CD28 domain to drive T cell activation, resulting in cytokine secretion, T cell proliferation, and cytolytic activity.

We revealed that effector T cells redirected by a tumor-specific chimeric antigen receptor are more effectively repressed by Treg cells when they are activated through a combined CD3zeta-CD28 signal compared to a CD3zeta signal without CD28 costimulation. Mutations in the CD28 signaling domain of the chimeric antigen receptors resulted in abolished IL-2 secretion by prevention of CD28 mediated lck activation. Abolished IL-2 induction in redirected effector T cells expressing the modified CD3zetaCD28delta antigen receptor increased their in vivo efficacy in an anti-tumor response by reduced sustaining of Treg cell suppression.

While data indicate the dichotomy of CD28 costimulation in inducing full effector T cell activation and sustaining Treg repression, our findings provide a strategy to improve the efficacy of the T cell anti-tumor attack in the presence of Treg cells for use in adoptive immunotherapy of cancer.

No relevant conflicts of interest to declare.