The role of the Fc receptor (FcR) of thymus-derived lymphocytes. I. Presence of FcR on cytotoxic lymphocytes and absence of direct role in cytotoxicity

FcγRIIB expressed on CD8+ T cells limits responsiveness to PD-1 checkpoint inhibition in cancer

Checkpoint inhibition using Fc-containing monoclonal antibodies has emerged as a powerful therapeutic approach to augment antitumor immunity. We recently showed that FcγRIIB, the only inhibitory IgG-Fc receptor, is expressed on a population of highly differentiated effector CD8+ T cells in the tumors of mice and humans, raising the possibility that CD8+ T cell responses may be directly modulated by checkpoint inhibitor binding to T cell-expressed FcγRIIB. Here, we show that despite exhibiting strong proliferative and cytokine responses at baseline, human FcγRIIBpos CD8+ T cells exhibited reduced responsiveness to both PD-1 and CTLA-4 checkpoint inhibition as compared with FcγRIIBneg CD8+ T cells in vitro. Moreover, frequencies of FcγRIIBpos CD8+ T cells were reduced after treatment of patients with melanoma with nivolumab in vivo. This reduced responsiveness was FcγRIIB dependent, because conditional genetic deletion of FcγRIIB on tumor-specific CD8+ T cells improved response to checkpoint blockade in B16 and LLC mouse models of cancer. The limited responsiveness of FcγRIIBpos CD8+ T cells was also dependent on an intact Fc region of the checkpoint inhibitor, in that treatment with Fc-devoid anti-PD-1 F(ab) fragments resulted in increased proliferation of FcγRIIBpos CD8+ T cells, without altering the response of FcγRIIBneg CD8+ T cells. Last, the addition of FcγRIIB blockade improved efficacy of PD-1 checkpoint inhibition in mouse models of melanoma, lung, and colon cancer. These results illuminate an FcγRIIB-mediated, cell-autonomous mechanism of CD8+ T cell suppression, which limits the efficacy of checkpoint inhibitors during antitumor immune responses in vivo.

FcγRs in health and disease

Genetic defects affecting the humoral immune response and especially the production of antibodies of the immunoglobulin G (IgG) isotype result in a heightened susceptibility to infections. Studies over the last years have demonstrated the crucial role of Fc-receptors for IgG (FcγRs) widely expressed on innate immune effector cells in mediating the protective function of IgG. During the last years, additional ligands interacting with FcγRs as well as additional receptors binding to IgG glycosylation variants have been identified. In this review, we discuss how the interaction of these different ligands with classical and novel Fcγ-receptors influences the immune response and which strategies microorganisms have developed to prevent them.

Fcgamma receptors as regulators of immune responses

In addition to their role in binding antigen, antibodies can regulate immune responses through interacting with Fc receptors (FcRs). In recent years, significant progress has been made in understanding the mechanisms that regulate the activity of IgG antibodies in vivo. In this Review, we discuss recent studies addressing the multifaceted roles of FcRs for IgG (FcgammaRs) in the immune system and how this knowledge could be translated into novel therapeutic strategies to treat human autoimmune, infectious or malignant diseases.

Structural bases of Fc gamma R functions

This review describes structures which determine the biological activities triggered by Fc gamma R and account for the cell-mediated functions of IgG antibodies in physiology and pathology. The binding specificity and affinity of Fc gamma R depend primarily on IgG-binding structures, in their immunoglobulin-like extracellular domains. Binding is however also influenced by subunits that associate to multichain Fc gamma R. Effector and regulatory intracytoplasmic sequences that are unique to molecules of the Fc gamma RIIB family determine the internalization properties of these receptors. Immunoreceptor Tyrosine-based Activation Motifs (ITAMs) are intracytoplasmic effector sequences shared by Fc gamma R and other receptors involved in the recognition of antigen, which trigger cell activation and internalization. Immunoreceptor Tyrosine-based Inhibition Motifs (ITIMs) are intracytoplasmic sequences, shared by Fc gamma RIIB and a growing number of negative coreceptors which negatively regulate cell activation via ITAM-bearing receptors. Altogether, these structures enable IgG antibodies to exert a variety of finely tuned biological effects during the immune response.

The same tyrosine-based inhibition motif, in the intracytoplasmic domain of Fc gamma RIIB, regulates negatively BCR-, TCR-, and FcR-dependent cell activation

The cell-triggering properties of BCR, TCR and FcR depend on structurally related immunoreceptor tyrosine-based activation motifs (ITAMs). Fc gamma RIIB have no ITAM and do not trigger cell activation. When coaggregated to BCR, they inhibit B cell activation. We show here that, when coaggregated to these receptors, Fc gamma RIIB inhibit Fc epsilon RI-, Fc gamma RIIA-, and TCR-dependent cell activation. Inhibition also affected cell activation by single ITAMs, in isolated FcR or TCR subunits. The same tyrosine-based inhibitory motif (ITIM), which is highly conserved in murine and human Fc gamma RIIB and that was previously shown to inhibit BCR-dependent B cell activation, was required to regulate TCR- and FcR-dependent cell activation. Our findings endow Fc gamma RIIB, and thus IgG antibodies, with general immunoregulatory properties susceptible to act on all ITAM-containing receptors.

The presence of a receptor for complement on T lymphocytes. Restriction of the complement receptor to Fc-receptor-bearing T lymphocytes

Using rhodaminated guinea pig anti-ovalbumin:ovalbumin:complement complexes, a fluoresceinated monoclonal anti-Thy 1 antibody and a FACS-II equipped with dual fluorescence detection channels, we find that 25-30% of Thy 1+ splenocytes express a receptor for complement. That a receptor for complement is involved in binding the guinea-pig AgAb:C' complexes is supported by the observations that: a) guinea-pig complexes, which were not treated with a serum source of complement or which were treated with either fresh serum in the presence of EDTA or with heat-inactivated serum, do not bind to the T lymphocytes, and b) heat-aggregated human gamma globulin, which effectively inhibits binding of mouse AgAb complexes to the aFcR gamma, has no effect on the binding of guinea-pig AgAb:C' complexes to the T lymphocytes. By analyzing cell subpopulations isolated by cell sorting, it is demonstrated that the C'R-positive T lymphocyte clearly delineates a major subpopulation of aFcR gamma-positive T lymphocytes, whereas no cells are found bearing a C'R, while lacking the aFcR gamma. The implications of the presence of a C'R in immunoregulation are discussed.

Expression of theta antigen on mouse thymocytes during the cell cycle

Cells from normal mouse thymus were subdivided according to size and analysed for surface antigen expression, protein content and DNA expression and content. It could be shown, that the expression of theta-antigen followed protein accumulation in individual cells. Protein content on the other hand, was correlated with DNA synthesis even if it varied over a greater range. Only one subgroup of small non-DNA-synthesizing cells was calculated to have a higher density indicative of a more condensed state together with an increased surface antigen content. These cells might represent the G0 state of the remnant proliferating population and most resembled the small cortical cells.

Mechanisms of cell-mediated cytotoxicity in mice rejecting xenogeneic human lymphoblastoid cells

The in vivo immunization of mice with human lymphoblastoid cell line LNH13 generates direct cell-mediated cytotoxicity by spleen cells. The lytic activity appears as early as day 3 after the intraperitoneal inoculation of 7.5 x 10(6) cells and persists at least until day 11. The killer cells do not adhere to plastic and are not retained on nylon wool columns or on Degalan beads coated with mouse Ig plus rabbit-anti-mouse Ig. The effector cells are partly inhibited by treatment with anti-Thy-1.2 serum plus complement, but this inhibition appears to be non-specific since anti-serum alone or normal serum plus complement have the same effects. Heat-aggregated IgG strongly inhibits cytotoxicity, indicating that the effector cells are Fc-positive and that such receptors are implicated in lysis. Altogether, these features strongly argue for an ADCC phenomenon. The involvement of antibodies is demonstrated by the fact that eluates (56 degrees C, 30 min) from immune cells alone induce lysis in the presence of normal spleen cells as effectors. The lytic activity of these eluates can be removed by specific adsorption on protein A coupled to Sepharose beads and on the human lymphoid target cells. Positive complementation between immune and non-immune spleen cells suggest that the arming process may occur in vitro during the assay, when antibodies are released by plasmacytes.