CD6: A New Target for Cancer Immunotherapy
David A. Fox
Rogel Cancer Center and Clinical Autoimmunity Center of Excellence
University of Michigan
Feng Lin
Lerner Research Institute
Cleveland Clinic
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D6 is a cell surface glycoprotein on human lymphocytes, including T cells and natural killer (NK) cells. CD6 is becoming an attractive therapeutic target for many autoimmune diseases because genetic absence of CD6 or treatment with an anti-CD6 monoclonal antibody (mAb) protects against and even reverses autoimmunity. Here, we describe our recent studies strongly suggesting that anti-CD6 should also be evaluated as a new cancer immunotherapy.

The major limitation of what are termed “checkpoint inhibitor” immunotherapies for cancer is induction of autoimmune toxicities. An anti-CD6 mAb termed UMCD6 differs from currently available cancer immunotherapy mAbs in its unique dual ability to stimulate lymphocytes to attack and kill cancer cells of many types while concurrently preventing other lymphocytes from inducing autoimmune conditions. In addition, CD6-targeted antibody-drug conjugates (CD6-ADCs) could be used to treat blood cancers in which the tumor cells themselves express CD6, such as T cell lymphoma, certain chronic lymphocytic leukemias, and NK cell lymphoma.

CD6 and Its Ligands

CD6 is found on the surface of most mature T cells, a small percentage of B cells, and about 50% of NK cells. CD6 is a receptor for CD166/ALCAM (activated leukocyte cell adhesion molecule) and for CD318 (CDCP1, CUB-domain-containing protein 1). Both of these molecules are strongly expressed by the majority of cancer cells. Both CD166 and CD318 are engaged in CD6-dependent adhesion of T lymphocytes. Some of the cell surface CD318 is shed from the cell membrane, creating soluble CD318 (sCD318). sCD318 is found in rheumatoid arthritis (RA) joint fluid at levels higher than in normal or RA serum. At concentrations similar to this in vivo gradient, sCD318 is chemotactic for T lymphocytes and is therefore partly responsible for attracting these cells into inflamed joints. Cancer cells can also shed CD318, and the level of sCD318 may determine whether lymphocytes are attracted into the tumor microenvironment.

CD6 and Autoimmune Disease

CD6 deficiency has not been reported in humans, but CD6 polymorphisms are linked to various human autoimmune diseases, notably multiple sclerosis (MS). No anti-CD6 mAbs are currently approved for clinical use in the US, but an anti-CD6 mAb is approved for treatment of patients with psoriasis and psoriatic arthritis in India. We recently showed a critical role of CD6 in regulating CD4+ T cell responses in mouse models of MS, uveitis, and RA, using CD6-/- mice and CD6-humanized mice (which are genetically altered to express human CD6 instead of mouse CD6). We identified UMCD6, a mouse anti-human CD6 monoclonal antibody (mAb), as a potential therapeutic mAb for these diseases, that binds and internalizes CD6 without depleting lymphocytes. We have also created a “humanized” form of UMCD6 that would reduce the incidence of infusion reactions in future clinical trials.
Table 1. Properties of UMCD6
Unique compared to other anti-CD6 mAbs due to the CD6 epitope that it recognizes (i.e., the short amino acid sequences of the protein that is recognized by the immune system, especially antibodies, and induces a direct/potent immune response) and its very high affinity.
UMCD6 rapidly caps and internalizes CD6, and it therefore does not deplete lymphocytes by complement fixation or Fc receptor engagement -> unlikely to impair host defenses against infection.
Its target, CD6, is only expressed on lymphocytes 🡪 no off-target toxicity is expected. Moreover, CD6-/- mice are healthy.
Humanized UMCD6 performs comparably to parent mAb and is likely to minimize immunogenicity, allergic reactions, or tachyphylaxis.
UMCD6 directly activates NK and CD8+ T cells that kill cancer cells, even in the absence of CD4+ T cells (unlike currently used checkpoint inhibitors).
Outstanding results in multiple mouse models of human autoimmune disease

  • potential for efficacy in all T cell-mediated autoimmune diseases: RA, spondyloarthropathies, juvenile rheumatoid arthritis, MS, psoriasis/psoriatic arthritis, uveitis, and checkpoint inhibitor toxicities.
CD6-ADC developed by conjugating a clinically proven mitotic toxin onto UMCD6 represents a new drug candidate for treating certain blood cancers such as T cell lymphomas.

Targeting CD6 to Treat Cancer

We also tested the effects of UMCD6 on the ability of human lymphocytes to kill cancer cells. Co-culture experiments using a multiplexed time-lapsed imaging system, including cell lines derived from human triple-negative breast cancer, non-small-cell lung cancer, and prostate cancer, showed substantial enhancement of cancer cell death and reduced survival of cancer cells in the presence of human lymphocytes and UMCD6.

The augmentation of lymphocyte tumor killing activity by UMCD6 was mainly due to direct effects of this mAb on cytotoxic lymphocytes, especially NK cells and CD8+ T cells. Moreover, UMCD6 exerted similar effects in vivo in a human breast cancer xenograft system in immunodeficient mice. These anticancer effects are accompanied by and depend on an extensive change in the program of gene expression in cytotoxic lymphocytes, including increased expression of stimulatory receptors and downregulation of inhibitory receptors, changes induced directly by UMCD6.

The anticancer effects of UMCD6 differ mechanistically from the mechanisms employed by conventional checkpoint inhibitor mAbs currently used in cancer immunotherapy, including anti-PD-1, anti-PD-L1, and anti-CTLA-4 mAbs. Those antibodies activate CD4+ lymphocytes, which then participate in the activation of cytotoxic lymphocytes. In contrast, UMCD6 directly augments NK cells and cytotoxic CD8+ cells.

Importantly, conventional checkpoint inhibitors induce significant autoimmune/inflammatory toxicity in many organs, which limits the intensity and duration of immunotherapy. Humans or mice that lack PD-1 or CTLA-4 correspondingly exhibit a global autoimmune diathesis that parallels the range of toxicities observed when these structures are targeted in immunotherapy of cancer. In contrast, CD6-/- mice are healthy, breed normally, and are resistant to the induction of autoimmune diseases that are driven by Th1 or Th17 cells. Moreover, in CD6-humanized mice, UMCD6 prevents or reverses multiple models of human autoimmune disease.

ADC Targeting CD6 to Directly Kill Blood Tumors Expressing CD6

We found that CD6 is expressed at high levels in certain blood cancers including T cell lymphoma (TCL), suggesting that CD6 can be targeted to directly kill these tumor cells. We have therefore developed a CD6-ADC by conjugating an FDA-approved mitotic toxin, monomethyl auristatin E (MMAE), onto UMCD6.

By combining the selectivity of UMCD6 for T and NK cells, and the selectivity of the mitotic toxin MMAE for actively dividing cells, a low dose of a CD6-ADC selectively kills the TCL cells that are actively proliferating, while sparing normal, quiescent CD6+ T cells and NK cells, and other tissue cells that are proliferating but lack expression of CD6. We found that CD6-ADC is effective in shrinking developed tumors in pre-clinical models of TCL.

Conclusion

The concept that resuscitation of anticancer immunity must inevitably be accompanied by autoimmunity underestimates the versatility of the immune system, which has evolved to execute multiple and diverse tasks concurrently. We have described a unique dual effect of UMCD6 to both suppress autoimmune diseases through its effects on effector CD4+ T cells, while also activating the anticancer cytotoxic properties of NK cells and other cytotoxic lymphocytes.

This dual effect creates the potential for an approach to cancer immunotherapy that would, distinct from currently available checkpoint inhibitors, suppress rather than instigate serious autoimmune diseases, thus overcoming the major current limitation to the success of immune checkpoint inhibition in the treatment of human cancer. We also developed a CD6-ADC that selectively and directly kills CD6+ tumor cells while sparing normal quiescent T and NK cells even when they express CD6. These CD6-targeted agents hold promise as novel anticancer drugs.

References available upon request.
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