Cryopreservation of MHC multimers: Recommendations for quality assurance in detection of antigen specific T cells.

Fluorescence-labeled peptide-MHC class I multimers serve as ideal tools for the detection of antigen-specific T cells by flow cytometry, enabling functional and phenotypical characterization of specific T cells at the single cell level. While this technique offers a number of unique advantages, MHC multimer reagents can be difficult to handle in terms of stability and quality assurance. The stability of a given fluorescence-labeled MHC multimer complex depends on both the stability of the peptide-MHC complex itself and the stability of the fluorochrome. Consequently, stability is difficult to predict and long-term storage is generally not recommended. We investigated here the possibility of cryopreserving MHC multimers, both in-house produced and commercially available, using a wide range of peptide-MHC class I multimers comprising virus and cancer-associated epitopes of different affinities presented by various HLA-class I molecules. Cryopreservation of MHC multimers was feasible for at least 6 months, when they were dissolved in buffer containing 5-16% glycerol (v/v) and 0.5% serum albumin (w/v). The addition of cryoprotectants was tolerated across three different T-cell staining protocols for all fluorescence labels tested (PE, APC, PE-Cy7 and Quantum dots). We propose cryopreservation as an easily implementable method for stable storage of MHC multimers and recommend the use of cryopreservation in long-term immunomonitoring projects, thereby eliminating the variability introduced by different batches and inconsistent stability.

High frequency of T cells specific for cryptic epitopes in melanoma patients.

A number of cytotoxic T-cell epitopes are cryptic epitopes generated from non-conventional sources. These include epitopes that are encoded by alternative open reading frames or in generally non-coding genomic regions, such as introns. We have previously observed a frequent recognition of cryptic epitopes by tumor infiltrating lymphocytes isolated from melanoma patients. Here, we show that such cryptic epitopes are more frequently recognized than antigens of the same class encoded by canonical reading frames. Furthermore, we report the presence of T cells specific for three cryptic epitopes encoded in intronic sequences, as a result of incomplete splicing, in the circulation of melanoma patients. One of these epitopes derives from antigen isolated from immunoselected melanoma 2 (AIM2), while the two others are encoded in an alternative open reading frame of an incompletely spliced form of N-acetylglucosaminyl-transferase V (GNT-V) known as NA17-A. We have detected frequent T-cell responses against AIM2 and NA17-A epitopes in the blood of melanoma patients, both prior and after one round of in vitro peptide stimulation, but not in the circulation of healthy individuals and patients with breast or renal carcinoma. In summary, our findings indicate that the T-cell reactivity against AIM2 and NA17-A in the blood of melanoma patients is extensive, suggesting that-similar to melan A (also known as MART1)-these antigens might be used for immunomonitoring or as model antigens in several clinical and preclinical settings.