Doctor of Philosophy
Microbiology and Immunology
CD5 is a member of scavenger receptor cysteine-rich superfamily that is expressed primarily on T cells. It can attenuate T-cell receptor signaling and impair cytotoxic T lymphocyte (CTL) activation and is a therapeutic targetable tumour antigen expressed on leukemic T and B cells. However, the potential therapeutic effect of functionally blocking CD5 to increase T cell anti-tumour activity against tumours (including solid tumours) has not been explored. CD5- solid tumours in CD5 knockout mice display increased in anti-tumour immunity. Hence, blocking CD5 function may have a potential therapeutic effect by enhancing CTL function. Here, I assessed CD5 levels in T cell subsets in different organs in mice bearing syngeneic 4T1 breast tumour homografts and determined the association between CD5 and increased CD69 and PD-1 (markers of T cell activation and exhaustion) by flow cytometry. I report that CD5 levels in T cells was higher in peripheral organs than in lymphocytes infiltrated into tumours, and that CD5high T cells from peripheral organs exhibited higher levels of activation and exhaustion than CD5low T cells from the same organs. Interestingly, among the population of tumour-infiltrated lymphocyte subtypes, CD8+ T cells with low CD5 were activated to a higher level than CD8+ T cells with high CD5, with concomitantly increased exhaustion markers. Thus, differential CD5 levels among T cells in tumours and lymphoid organs can be associated with different levels of T cell activation and exhaustion, suggesting that CD5 could be a therapeutic target for immunotherapeutic activation in cancer therapy. I then studied the effect on primary T cell effector function of targeting CD5 ex vivo using an anti-CD5 MAb. The result showed enhanced cytotoxic T cell capacity to respond to activation and enhanced the capacity of CD8+ T cells to kill 4T1-mouse tumour cells in an ex vivo assay. These data support the potential of blockade of CD5 function to enhance T cell-mediated anti-tumour immunity. Based on these results I tested anti-CD5 in vivo as a single agent and in combination with other drugs and found a significant increase in activation and effector function of T cells: an effect that resulted in decreased 4T1 tumour homograft growth in vivo. These data suggest potential use of anti-CD5 MAb to enhance immune activation to poorly immunogenic tumour antigens and reduce tumour growth.
Summary for Lay Audience
T cells are an important part of the immune system, containing molecules that participate in recognizing foreign substances (antigens) including antigens that arise in cancer cells. When antigens stimulate those molecules, they activate T cells to recognize and kill the foreign organisms that contain those antigens (including cancer cells that contain unusual molecules that act as antigens). On the other hand, T cells also contain proteins that control their degree of activation by limiting their response to antigen recognitions: those proteins are termed “inhibitory receptors”. CD5 is such a receptor, expressed mainly on T cells, that limits T cell capacity to undergo activation in response to recognition of tumour antigen. Other studies have shown that mice with deleted CD5 and intact immune systems had delayed growth of transplanted tumours, suggesting that loss of CD5 can lead to increased immune activation in response to cancer cell antigen recognition, and immune cell-mediated attack and destruction of cancer cells resulting in slower cancer growth. I propose that blocking CD5 will coax T cells into better anti-cancer action, both as a treatment applied alone and in combination with already-approved immunotherapies. In this thesis, I describe my study of the correlation between CD5 level and T cell activation and exhaustion, and my assessment of dynamic CD5 expression among peripheral organs. I also inhibited CD5 on T cells obtained from mouse spleens using an anti-CD5 blocking antibody, a step taken to make them more active in recognizing and killing cancer cells. I found a significant increase in T cell effector function following treatment with a blocking anti-CD5 antibody and, based on these results, I then moved on to test the efficacy of this drug to treat mice with fully functional immune systems and transplanted mouse breast tumours. Treating mice with anti-CD5 antibody increased T cell activation function in spleen, lymph node and tumour. Furthermore, tumour growth was delayed by the treatment. These findings suggest that treatment with anti-CD5 MAb can result in increased T cell effector function and reduced tumour homograft growth. Further investigation of reduction of CD5 in combination with other immunotherapy approaches may enhance anti-cancer immune T cells.
Alotaibi, Faizah, "Targeting CD5 to enhance immune T cell activation and function in treatment of solid tumours" (2020). Electronic Thesis and Dissertation Repository. 7669.