Antibodies that coengage activating FcγRs expressed by tumor-associated leukocytes facilitate the selective elimination of intratumoral T cells.

Antibodies targeting checkpoint inhibitors or co-stimulatory receptors on T cells have shown significant antitumor efficacy in preclinical and clinical studies. In mouse tumor models, engagement of activating Fcc receptor (FccR)-expressing immune cells was recently shown to be required for the tumoricidal activity of antibodies recognizing the tumor necrosis factor superfamily receptor (TNFR) GITR (CD357) and CTLA-4 (CD152). In particular, activating FccRs facilitated the selective elimination of intratumoral T-cell populations. However, it remains unclear whether FccRs contribute to the antitumor efficacy of other immunomodulatory antibodies. Here, we explored the mechanism of antitumor activity mediated by an agonistic antibody (clone OX86) to the co-stimulatory TNFR OX40 (CD134). OX40 was highly expressed by intratumoral T cells, particularly those of the FoxP3 þ regulatory T-cell (Treg) lineage. OX86 administration resulted in the depletion of intratumoral regulatory T cells in an activating FccR-dependent manner, which correlated with tumor regression. Together with previous data from our group and others, these findings support a mechanism whereby antibodies targeting antigens highly expressed by intratumoral T cells can mediate their elimination by FccR-expressing immune cells, and facilitate subsequent antitumor immunity. Agonistic antibodies targeting co-stimulatory tumor necrosis factor superfamily receptors (TNFRs) expressed by T cells, such as GITR (CD357) or OX40 (CD134), have been shown to enhance the proliferation and activation of isolated T cells. [1][2][3][4] Moreover, in preclinical tumor efficacy studies, these agonistic antibodies have shown potent tumoricidal activity. [5][6][7] The current model stipulates that agonistic antibodies targeting GITR, OX40 and other costimulatory receptors on T cells directly enhance the cytotoxic effector function of T cells, while counteracting the immunosuppressive effects of immune-regulatory populations, such as regulatory T cells (Tregs). [8][9][10][11] We recently reported that an agonistic antibody targeting GITR (DTA-1) requires the engagement of activating FcgRs but not the inhibitory FcgR (FcgRIIB) to mediate antitumor activity in mice. 12 This finding challenges the notion that FcgRIIB-mediated crosslinking is generally required for the in vivo activities of agonistic antibodies targeting TNFRs. [13][14][15] Importantly, DTA-1 administration triggered the rapid and selective elimination of intratumoral T cells, particularly those of the Treg lineage, as defined by FoxP3 expression. The extent of Treg depletion in the tumor microenvironment directly correlated with the antitumor efficacy. A similar mechanism was observed for antibodies targeting the non-TNFR immunoregulatory molecule CTLA-4. 12,16 A common feature of GITR and CTLA-4 is their high expression by intratumoral populations of T cells, with Tregs expressing the highest levels of each receptor. 12,17,18 Here, we investigated whether antibody-mediated intratumoral T-cell depletion might con...

Retroelements are important evolutionary forces but can be deleterious if left uncontrolled. Members of the human APOBEC3 family of cytidine deaminases can inhibit a wide range of endogenous, as well as exogenous, retroelements. These enzymes are structurally organized in one or two domains comprising a zinc-coordinating motif. APOBEC3G contains two such domains, only the C terminal of which is endowed with editing activity, while its N-terminal counterpart binds RNA, promotes homo-oligomerization, and is necessary for packaging into human immunodeficiency virus type 1 (HIV-1) virions. Here, we performed a large-scale mutagenesisbased analysis of the APOBEC3G N terminus, testing mutants for (i) inhibition of vif-defective HIV-1 infection and Alu retrotransposition, (ii) RNA binding, and (iii) oligomerization. Furthermore, in the absence of structural information on this domain, we used homology modeling to examine the positions of functionally important residues and of residues found to be under positive selection by phylogenetic analyses of primate APOBEC3G genes. Our results reveal the importance of a predicted RNA binding dimerization interface both for packaging into HIV-1 virions and inhibition of both HIV-1 infection and Alu transposition. We further found that the HIV-1-blocking activity of APOBEC3G N-terminal mutants defective for packaging can be almost entirely rescued if their virion incorporation is forced by fusion with Vpr, indicating that the corresponding region of APOBEC3G plays little role in other aspects of its action against this pathogen. Interestingly, residues forming the APOBEC3G dimer interface are highly conserved, contrasting with the rapid evolution of two neighboring surface-exposed amino acid patches, one targeted by the Vif protein of primate lentiviruses and the other of yet-undefined function.