Resolving conflicting data on expression of the Tn antigen and implications for clinical trials with cancer vaccines

Abstract

The tumor-associated Tn antigen has been investigated extensively as a biomarker and therapeutic target. Cancer vaccines containing the Tn antigen as a single tumor antigen or as a component of a polyvalent vaccine have progressed into phase I and II clinical trials. One major focus of Tn-based vaccines is the treatment of prostate cancer patients. Although expression of the antigen on prostate tumors is a critical prerequisite, previous reports investigating Tn expression in prostate tumors have produced conflicting results. Using a combination of immunohistochemistry and carbohydrate microarray profiling, we show that only 4% to 26% of prostate tumors express the Tn antigen. Based on our results, the majority of prostate cancer patients do not express the appropriate antigen. Therefore, efforts to preselect the subset of prostate cancer patients with Tn-positive tumors or apply Tn vaccines to other cancers with higher rates of antigen expression could significantly improve clinical response rates. Because conflicting information on carbohydrate expression is a general problem for the field, the approach described in this article of analyzing antigen expression with multiple antibodies and using carbohydrate microarray profiles to interpret the results will be useful for the development of other carbohydrate-based cancer vaccines and diagnostics.

Figure 1

Structures of the Tn antigen and related carbohydrates

Figure 2. Immunohistochemical staining of human tissues

Left column: a typical positive staining of prostate cancer tissue by the antibodies (40X, scale bar = 20 μm). Middle-left column: a typical positive staining of prostate cancer tissue by the antibodies (10X, scale bar = 100 μm). Middle right column: a typical negative staining of prostate cancer tissue by the antibodies (10X, scale bar = 100 μm). Right column: positive staining of colon cancer tissue by the antibodies (10X, scale bar = 100 μm).

Figure 3. Carbohydrate Microarray Profiles of 1E3, HBTn-1, B1.1, and PolyTn

Antibody binding was evaluated on a carbohydrate microarray and displayed in the form of a heat map with data for each antibody in columns and each carbohydrate in rows. Fluorescence values have been converted to log base 2. Signals are represented as colored boxes ranging from black (saturated signal) to white (signal less than 2 fold higher than the background; ≤8.0). Data are at the following antibody concentrations: 1E3 (1:500), HBTn-1 (1:80), B1.1 (1:10), and PolyTn (1:1000). Tabulated data can be found in the supporting information.