As the “predecessors” in immune checkpoint inhibitors, the development of anti-CTLA-4 monoclonal antibodies in recent years has not been ideal. From the perspective of approved indications, their single-drug treatment uses are few and can only be paired with PD-1/L1 inhibitors. If we look at the failure cases of clinical research, it will be even more heart-wrenching.
So the question is, what is blocking the anti-CTLA-4 monoclonal antibody? The research findings just published by the Rockefeller University team in Cancer Immunology Research provide an explanation: the inhibitory IgG receptor FcγRIIB can act as an immune checkpoint to inhibit antibody-dependent cell-mediated cytotoxicity/phagocytosis (ADCC/ADCP) by tripping down anti-CTLA-4 monoclonal antibodies, resulting in the inability of anti-CTLA-4 monoclonal antibodies to effectively clear regulatory T cells, greatly reducing their efficacy.
Previous basic studies have shown that regulatory T cells within the human tumor microenvironment generally exhibit high expression of CTLA-4, and the activation of the ADCC/ADCP effect by the Fc segment of anti-CTLA-4 monoclonal antibodies to clear regulatory T cells is crucial for their anti-tumor activity. However, the replication of this effect in human patients is not ideal.
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| CTLA4-2233HAF555 | Recombinant Human CTLA4 Protein, Fc/His-tagged, Alexa Fluor 555 conjugated | Human | HEK293 | Fc/His |
| PDCD1-172H | Active Recombinant Human PDCD1, no tag | Human | HEK293 | N/A |
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The research team at Rockefeller University believes that the reason for the above situation may be the differences between human cancer patients and mouse models used in basic research, especially the Fc corresponding to the segment of monoclonal antibodies Fcγ Receptor. The structure, function, and expression levels of FcγRs may vary greatly between humans and mice, hence the decision to use CTLA-4 and FcγR humanized mouse model is used to evaluate whether the whole human anti-CTLA-4 monoclonal antibody can clear regulatory T cells in tumors. If not, the reason must be clarified.
Preliminary experiments have shown that the humanized mouse model of CTLA-4/FcγRs constructed by researchers can basically reproduce the tumor and immune infiltration of human cancer patients; However, treating mice with the Fc segment of two approved anti-CTLA-4 monoclonal antibodies, ipilimumab or Tremelimumab, did not effectively clear regulatory T cells. Their anti-tumor effects mainly come from the activation of CD8+T cells, which is consistent with the current clinical practice.
But when the surface of regulatory T cells clearly shows CTLA-4 expression, why can’t anti-CTLA-4 monoclonal antibodies take them? Researchers also used the Fc segment of ipilimumab to construct antibodies targeting CD4, and found that even if the target was changed, there was not much killing power. Therefore, the problem lies in the tumor microenvironment, where there is a mechanism that limits the killing effect of the Fc segment of the monoclonal antibody in activating ADCC/ADCP. Therefore, the first step is to look up the FcγRs combined with the Fc segment.
Flow cytometry analysis showed that a large portion of immune cells infiltrating tumors in humanized mouse models showed expression of FcγRIIB on their surface. However, combining anti-CTLA-4 monoclonal antibodies with monoclonal antibodies that block FcγRIIB, or adjusting the Fc segment of existing anti-CTLA-4 monoclonal antibodies to weaken their affinity for FcγRIIB, can significantly enhance the ability of anti-CTLA-4 monoclonal antibodies to clear regulatory T cells. With fewer regulatory T cells, the anti-tumor immune intensity increases.
However, researchers are also aware that FcγRIIB is commonly present in various immune cells in the human body, and targeting it directly is not appropriate. Existing anti-CTLA-4 monoclonal antibodies also have significant side effects in clinical use, and it is not cost-effective to modify their Fc segments to better target FcγRIIB. Therefore, the researchers suggest using the chemokine receptor CCR8 as a target to selectively clear regulatory T cells infiltrating tumors, trying to avoid harming the innocent.
So the final step of the experiment was very interesting. Researchers found that in the humanized mouse model of CTLA-4/FcγRs, anti-CCR8 monoclonal antibodies must undergo the same Fc segment modification before, that is, to weaken the affinity for FcγRIIB and enhance the affinity for activated FcγRs, to effectively clear regulatory T cells and mobilize anti-tumor immune responses.
It seems that FcγRIIB is not only a roadblock for anti-CTLA-4 monoclonal antibodies. Although PD-1/L1 inhibitors do not primarily rely on the activation of ADCC/ADCP effects by the Fc segment, the development of monoclonal antibody inhibitors for other immune checkpoialexants, especially targeting regulatory T cells, may require attention to the impact of FcγRIIB and specialized optimization of antibody structure.
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Reference
Knorr D A, Blanchard L, Leidner R S, et al. FcγRIIB is an immune checkpoint limiting the activity of Treg-targeting antibodies in the tumor microenvironment[J]. Cancer Immunology Research, 2023.
Vargas F A, Furness A J S, Litchfield K, et al. Fc effector function contributes to the activity of human anti-CTLA-4 antibodies[J]. Cancer Cell, 2018, 33(4): 649-663. e4.
Sharma A, Subudhi S K, Blando J, et al. Anti-CTLA-4 immunotherapy does not deplete FOXP3+ regulatory T cells (Tregs) in human cancers[J]. Clinical Cancer Research, 2019, 25(4): 1233-1238.
Kidani Y, Nogami W, Yasumizu Y, et al. CCR8-targeted specific depletion of clonally expanded Treg cells in tumor tissues evokes potent tumor immunity with long-lasting memory[J]. Proceedings of the National Academy of Sciences, 2022, 119(7): e2114282119.