Autologous tumor vaccines processed to express alpha-gal epitopes: a practical approach to immunotherapy in cancer

Abstract

This review describes a method by which the human natural anti-Gal antibody can be exploited as an endogenous adjuvant for targeting autologous tumor vaccines to antigen-presenting cells (APCs). Tumor cells remaining in the patient after completion of surgery, radiation, and chemotherapy are the cause of tumor relapse. These residual tumor cells can not be detected by imaging, but their destruction may be feasible by active immunotherapy. Since specific tumor-associated antigens (TAAs) have not been identified for the majority of cancers, irradiated autologous tumor vaccines have been considered as an immunotherapy treatment that may elicit an immune response against the residual tumor cells expressing TAAs. However, tumor cells evolve in cancer patients in a stealthy way, i.e., they are not detected by APCs, even in the form of vaccine. Effective targeting of tumor vaccines for uptake by APCs is a prerequisite for eliciting an effective immune response which requires transport of the vaccine by APCs from the vaccination site to the draining lymph nodes. In the lymph nodes, the APCs transporting the vaccine process and present peptides, including the autologous TAA peptides for activation of the tumor-specific T cells. The required targeting of vaccines to APCs is feasible in humans by the use of anti-Gal. This antibody interacts specifically with the alpha-gal epitope (Galalpha1-3Galbeta1-4GlcNAc-R) and is the only known natural IgG antibody to be present in large amounts in all humans who are not severely immunocompromised. The alpha-gal epitope can be synthesized on any type of human tumor cell by the use of recombinant alpha1,3galactosyltransferase (alpha1,3GT). Solid tumors obtained from surgery are homogenized and their membranes subjected to alpha-gal epitope synthesis. Similarly, alpha-gal epitopes can be synthesized on intact tumor cells from hematological malignancies. Administration of irradiated autologous tumor vaccines processed to express alpha-gal epitopes results in in situ opsonization of the vaccinating cells or cell membranes due to anti-Gal binding to these epitopes. The bound antibody serves to target the autologous tumor vaccine to APCs because the Fc portion of the antibody interacts with Fcgamma receptors on APCs. Since patients receive their own TAAs, the vaccine is customized for autologous TAAs in the individual patient. The repeated vaccination with such autologous tumor vaccines provides the immune system of each patient with an additional opportunity to be effectively activated by the autologous TAAs. In some of the immunized patients this activation may be potent enough to induce an immune-mediated eradication of the residual tumor cells expressing these TAAs.

Fig. 1

Anti-Gal–mediated targeting of autologous tumor vaccines expressing α-gal epitopes to APCs. Anti-Gal of the vaccinated patient binds in situ to α-gal epitopes on vaccinating tumor cells or cell membranes. The subsequent interaction between the Fc portion of the bound anti-Gal and Fcγ receptors (FcγRs) on APCs induces phagocytosis of the vaccine into the APCs and thus the effective internalization of the tumor-associated antigens (TAAs) on the tumor membranes. The internalized TAAs are processed and the various immunogenic TAA peptides (circle, square, and triangle) are presented by the APCs in association with MHC class I or II molecules

Fig. 2

Processing of tumor cell membranes to express α-gal epitopes on asparagine-linked (N-linked) carbohydrate chains of glycoproteins. Synthesis of α-gal epitopes is achieved in a suspension containing neuraminidase and recombinant α-1,3-galactosyltransferase (rα1,3GT) in a two-step reaction: (1) removal of sialic acid (SA) from carbohydrate chains by neuraminidase to expose the penultimate N-acteyllactosamine residues (Galβ1-4GlcNAc-R), and (2) linking of galactose from the sugar donor uridine-diphosphate-galactose (UDP-Gal) to the exposed N-acetyllactosamine residues by rα1,3GT to form α-gal epitopes. The de novo synthesized α-gal epitopes readily bind the natural anti-Gal Ab

Fig. 3A–D

Anti-Gal–mediated targeting of freshly obtained B-lymphoma tumor cells by APCs. The lymphoma cells were processed to express α-gal epitopes and incubated with anti-Gal (10 μg/ml) and with macrophages (B) or dendritic cells (D). The elongated nucleus in (B) is that of the macrophage, and in (D) is that of the dendritic cell, whereas the round nuclei in (B) and (D) are of the lymphoma cells. The representative macrophage internalized five lymphoma cells and the dendritic cell internalized one lymphoma cell. Control lymphoma cells that were not processed to express α-gal epitopes and which were incubated with anti-Gal, were not internalized by macrophages (A), or dendritic cells (C) (×1,000). Reproduced from [78] with the publisher’s permission