A team of tumor immunologists in France and Switzerland has established an improved, reliable immune monitoring tool that allows for more accurate measurement of the number, subtype, and activity of cancer antigen-specific CD4+ T cells in cancer patients treated with therapeutic cancer vaccines. The new technology may significantly aid tumor immunologists around the world who are working to improve the effectiveness of this emerging and highly promising class of immune system-based cancer therapies.
In an article published this week in the Proceedings of the National Academy of Sciences, Cancer Research Institute investigators Maha Ayyoub, Ph.D., and Danila Valmori, Ph.D., and their team at the French National Institute of Health and Medical Research (INSERM) in Nantes, France, along with Danijel Djocinovic, Immanuel Luescher, Ph.D., and colleagues at the Lausanne Branch of the Ludwig Institute for Cancer Research Ltd, describe the successful generation of MHC class II tetramers to evaluate CD4+ T-cell responses in patients who received a therapeutic cancer vaccine in a previously reported study. That vaccine included recombinant NY-ESO-1 cancer antigen in Montanide ISA-51, an oil-and-water emulsion, given in combination with CpG 7909, a non-specific immune boosting adjuvant, to patients with cancers that express NY-ESO-1.
Clinical and laboratory studies have confirmed NY-ESO-1 expression in many different cancer types—including melanoma, lung, breast, and ovarian cancers—but not healthy tissues with the exception of testicular germ cells of normal adults, which are protected from recognition and attack by the immune system. Studies have also shown that the immune system is able to recognize and target the antigen naturally (i.e., spontaneous ESO+ immunity) or as a result of therapeutic cancer vaccination. Taken together, the cancer-specific expression of NY-ESO-1 and its strong immunogenicity make it a highly promising target for vaccine therapy.
Critical to the design of effective cancer vaccines is the accurate assessment of a vaccine's ability to elicit protective immune responses. Clinical studies have shown that therapeutic cancer vaccines are able to induce tumor antigen-specific CD4+ and CD8+ T cells and antibody-producing B cells. Further studies are required, however, to determine whether these immune responses protect cancer patients from recurrent disease.
Over the past decades, scientists have learned that there are several subsets of CD4+ T cells that serve distinct and opposing functions. For example, the "helper" sub-type (TH) produce interleukins that stimulate the proliferation and activity of other immune cells, including CD8+ T cells and antibody-producing B cells, and play a role in establishing immunological memory, or the ability of the immune system to recognize and mount a rapid immune response against a target it has already encountered. On the other hand, the "regulatory" sub-type (Treg) suppresses immune responses and can be detrimental to vaccine-induced anti-cancer immunity.
Scientists working to develop vaccine strategies that enhance the activity of CD4+ helper T-cell subtypes and inhibit suppressive subtypes must have at their disposal immune monitoring tools that distinguish the different types. In their PNAS paper, Ayyoub et al. describe their development of a new approach to this.
In recent years, tetramers—chemically engineered proteins containing four distinct subunits—have come into widespread use for immune monitoring, enabling scientists to quantify, isolate, and characterize the antigen-specific T-cell responses, such as those induced by a vaccine therapy. Conventional tetramer technology, however, is geared to measure primarily a subset of immune cells, the cytotoxic ("killer") CD8+ T cells, which recognize antigens presented by major histocompatibility complex (MHC) class I molecules. CD4+ T cells recognize antigens presented by MHC class II molecules, and are more difficult to detect by tetramers.
Previous attempts to establish CD4+ T-cell tetramers have proven challenging, due to factors including the greater degree of heterogeneity of MHC class II peptide complexes compared to refolded MHC class I molecules, and the weaker molecular binding ("low affinity") of MHC class II tetramers, leading to greater difficulties in detection. The difficulty of producing reliable MHC class II tetramers to identify antigen specific CD4+ T cells has been a major challenge for the field.
Prior to the establishment of the new tetramer, in order to gain a more complete picture of immune responses to cancer vaccination, scientists have been required to carry out immune tests that assess CD4+ T-cell activity by measuring levels of certain cellular products associated with their function, such as cytokines or chemokines.