3.3.2　Antibody Isotypes in Tumors

3.3.2　Antibody Isotypes in Tumors

Antibody functionality is determined by the constant fragment (Fc) and may be modulated by various post-translational modifications.From the perspective of antitumor cytotoxic responses, the human IgG1 antibody class is of primary importance, as these antibodies can bind to Fcγreceptor(FcγR) and trigger ADCC and antibody-mediated phagocytosis and mediate complement-based cytotoxicity.Furthermore, FcγR expressed on dendritic cells and macrophages may capture IgG-bound tumor antigens and present peptides derived from these antigens to T cells.Maturation of high-affinity antibodies and isotype switching to IgG1 may occur in TLS located within or in close to the tumor or in tumor-draining lymph nodes, and this results in the generation of plasma cells that may be released into the blood or reside locally and produce tumor-specific cytotoxic antibodies.The IgG antibody isotypes, and IgG1 in particular, are often associated with antitumor B cell activity.The correlation of MUC1-specific and Thomsen-Friedenreich-specific antibodies with longer survival is mainly associated with IgG isotypes.Tumor-specific IgG production by plasma cells present in metastases of patients with high-grade serous ovarian cancer was also associated with an antitumor response.Furthermore, effector and memory B cells expressing membrane IgG may also execute direct antitumor functions.

In hepatocellular carcinoma, it has been shown that IgG+ memory B cells concentrated in the tumor invasive margin can produce granzyme B and TRAIL.These cells also express surface markers characteristic of APCs, can produce IFNγ and can cooperate with CD8+ T cells, and are correlated with favourable prognosis.Likewise, several reports showed that B cell infiltration is associated with longer survival in patients with hepatocellular carcinoma.At the same time, accumulation of immunosuppressive B cells may be a negative factor in the same cancer type.Using human RNA-seq datasets from TCGA, we showed that a high proportion of IgG1 isotype RNA among all intratumorally expressed immunoglobulin heavy chain (IgH) sequences positively correlates with non-silent mutation burden in lung adenocarcinoma, further supporting the role of IgG1+ B cells in antigen presentation.Such a high IgG1 proportion was also strongly associated with longer survival in melanoma, KRAS-mutant lung adenocarcinoma, and non-papillary subtype of bladder cancer [51, 52].Figure 3.5 and Figure 3.6 describe intratumorally produced antibodies.By contrast, non-tumour-specific, antigen-free IgG1 antibodies do not occupy FcγRs, leaving them vacant and thus allowing IgG1 or IgG3 bound to the surface of tumour cells to trigger their recognition and elimination via ADCC and antibody-mediated phagocytosis.On the surface of tumour cells, avidity increases due to engagement of multiple receptors (Figure 3.7).Furthermore,allosteric increase of antigen-bound IgG1 affinity for FcγRs, still disputable, could enhance productive ADCC or phagocytic attack [53].

Figure 3.5　Intratumorally produced antibodies-1.
Source: Sharonov G V, Serebrovskaya E O, Yuzhakova D V, Britanova O V, Chudakov D M.“B Cells, Plasma Cells and Antibody Repertoires in the Tumour Microenvironment”.NatureReviewImmunology, 2020,20(5):294-307.

Figure 3.6　Intratumourally produced antibodies -2.
Source: Sharonov G V, Serebrovskaya E O, Yuzhakova D V, Britanova O V, Chudakov D M.“B Cells, Plasma Cells and Antibody Repertoires in the Tumour Microenvironment”.NatureReviewImmunology, 2020,20(5):294-307.(https://www.daowen.com)

Figure 3.7　Potential advantages of IgG1-biased over IgG3-biased humoral response.
Source: Sharonov G V, Serebrovskaya E O, Yuzhakova D V, Britanova O V, Chudakov D M.“B Cells, Plasma Cells and Antibody Repertoires in the Tumour Microenvironment”.NatureReviewImmunology, 2020,20(5):294-307.

In conclusion, the relative abundance of distinct antibody isotypes in the tumour and tumour-associated lymphoid structures is an important parameter of the tumour microenvironment.At the same time, the correlation of distinct antibody isotypes with poor or favourable prognoses or response to therapy does not allow one to discriminate the passive role of isotype as a biomarker of ongoing processes, in which antibody specificity is of low importance, and an active role in which antigen specificity determines the efficiency of cancer surveillance or promotion.

Notably, in the same human melanoma study of TCGA RNA-seq data, high proportions of IgA, IgD or IgE were associated with a poor prognosis.A high intra-tumoral proportion of the IgA isotype was also associated with negative prognosis in the KRAS-mutant subtype of lung adenocarcinoma.These observations are in line with those from several other publications.High intra-tumoral IgA level was found in various human cancers and is associated with shorter survival in patients with bladder cancer.In mouse models, IgA+ plasma cells expressing PDL1 and IL-10 were shown to impede T cell-dependent responses on immunogenic chemotherapy for prostate cancer.In general,the IgA isotype is often characteristic of the immunosuppressive B cells involved in the regulatory loop in which B cells promote expansion of regulatory T cell (Treg cell) populations, while Treg cells produce transforming growth factor-β (TGF-β), which mediates isotype class switching to IgA.This loop maintains gut homeostasis and may be naturally engaged by the immune system to regulate inflammation of various causes.For example, it has been shown that IgA+ B cells producing IL-10 and expressing PDL1 may accumulate in the liver during chronic inflammation and suppress cytotoxic CD8+ T lymphocytes capable of preventing hepatocellular carcinoma.Correspondingly, high intra-tumoral IgA expression levels may be considered as a biomarker of B cell involvement in the IL-10, TGFβ and Treg cell pathways, in at least some cancer types or subtypes.A high proportion of IgE may reflect high intra-tumoral IL-4 levels and TH cell polarization towards the TH2 cell fate, which is commonly associated with inefficient tumor control due to suppression of TH1 cell and cytotoxic T cell response.IgD antibodies have been shown to bind to basophils and can drive the production of factors such as IL-4, IL-5, IL-13, BAFF and APRIL, as a result, high IgD levels may be associated with IgE and IgA class switching and the generation of TH2 cells.These effects could potentially explain the correlation of a high proportion of intra-tumoral IgE and IgD with negative prognosis in melanoma [47, 51,52, 54].

It was shown that IgG4 antibodies, which are also associated with type 2 immunity, lack antitumoral effector functions and may block tumor-specific IgG1 response.A high proportion of IgG4 of all IgG in serum is associated with a poor prognosis in patients with melanoma, and tumor-specific IgG4 antibodies have been shown to be produced in situ by plasma cells located within the melanoma lesions.A high proportion of IgG4 has also been associated with favourable prognosis in some cancer types, such as lung squamous cell carcinoma, as well as in STK11mutant and proximal proliferative subtypes of lung adenocarcinoma.The explanation for this positive effect of IgG4 in particular tumor microenvironments remains unclear, but we hypothesize that it could be associated with the inability of IgG4 to form immune complexes, which could otherwise stimulate chronic inflammation and tissue remodelling processes and eventually induce immunosuppressive myeloid-derived suppressor cell phenotype [47, 55].

RNA-seq data have suggested that the relative proportions of IgG3 can be either neutral or negatively associated with survival in human melanoma43, although the reasons for this are unclear.This is in contrast with IgG1, which has a positive association with survival for this malignancy,even though both of these antibody isotypes are capable of triggering ADCC, phagocytosis,complement activation and antigen cross-presentation.A possible explanation could be that although IgG3 antibodies can interact with activating Fc receptors, there is evidence that the IgG3 response is early and transient and that IgG3 antibodies generally have a lower number of accumulated hypermutations and lower affinity.Faster turnover of IgG3 antibodies can also result in relatively lower local concentrations of IgG3 than IgG1, even if both isotypes are produced at comparable levels.An alternative explanation comes from the high affinity of monomeric IgG3 for FcγRI (which is expressed on macrophages) and FcγRIIIA (which is expressed on both macrophages and NK cells), and especially for the variant FcγRIIIA-158V.Therefore, non-tumorspecific, antigen-free IgG3 antibodies may efficiently occupy vacant FcγRs of NK cells and macro phages, thus blocking their interaction with cytotoxic antibodies bound to the surface of tumor cells.By contrast, non-tumor-specific, antigen-free IgG1 antibodies do not occupy FcγRs, leaving them vacant and thus allowing IgG1 or IgG3 bound to the surface of tumor cells to trigger their recognition and elimination via ADCC and antibody-mediated phagocytosis.On the surface of tumor cells, avidity increases due to engagement of multiple receptors.Furthermore, allosteric increase of antigen-bound IgG1 affinity for FcγRs, still disputable, could enhance productive ADCC or phagocytic attack.In conclusion, the relative abundance of distinct antibody isotypes in the tumor and tumor-associated lymphoid structures is an important parameter of the tumor microenvironment.At the same time, the correlation of distinct antibody isotypes with poor or favourable prognoses or response to therapy does not allow one to discriminate the passive role of isotype as a biomarker of ongoing processes, in which antibody specificity is of low importance,and an active role in which antigen specificity determines the efficiency of cancer surveillance or promotion.A way to look deeper into this picture is to analyze intra-tumoral immunoglobulin repertoires and clonality, as we discuss next [47, 56-59].