Label-free fluorescence lifetime imaging for the assessment of cell viability in living tumor fragments.

Significance: To enable non-destructive longitudinal assessment of drug agents in intact tumor tissue without the use of disruptive probes, we have designed a label-free method to quantify the health of individual tumor cells in excised tumor tissue using multiphoton fluorescence lifetime imaging microscopy (MP-FLIM). Aim: Using murine tumor fragments which preserve the native tumor microenvironment, we seek to demonstrate signals generated by the intrinsically fluorescent metabolic co-factors nicotinamide adenine dinucleotide phosphate [NAD(P)H] and flavin adenine dinucleotide (FAD) correlate with irreversible cascades leading to cell death. Approach: We use MP-FLIM of NAD(P)H and FAD on tissues and confirm viability using standard apoptosis and live/dead (Caspase 3/7 and propidium iodide, respectively) assays. Results: Through a statistical approach, reproducible shifts in FLIM data, determined through phasor analysis, are shown to correlate with loss of cell viability. With this, we demonstrate that cell death achieved through either apoptosis/necrosis or necroptosis can be discriminated. In addition, specific responses to common chemotherapeutic treatment inducing cell death were detected. Conclusions: These data demonstrate that MP-FLIM can detect and quantify cell viability without the use of potentially toxic dyes, thus enabling longitudinal multi-day studies assessing the effects of therapeutic agents on tumor fragments.

Optimizing Flow Cytometric Analysis of Immune Cells in Samples Requiring Cryopreservation from Tumor-Bearing Mice.

Most shared resource flow cytometry facilities do not permit analysis of radioactive samples. We are investigating low-dose molecular targeted radionuclide therapy (MTRT) as an immunomodulator in combination with in situ tumor vaccines and need to analyze radioactive samples from MTRT-treated mice using flow cytometry. Further, the sudden shutdown of core facilities in response to the COVID-19 pandemic has created an unprecedented work stoppage. In these and other research settings, a robust and reliable means of cryopreservation of immune samples is required. We evaluated different fixation and cryopreservation protocols of disaggregated tumor cells with the aim of identifying a protocol for subsequent flow cytometry of the thawed sample, which most accurately reflects the flow cytometric analysis of the tumor immune microenvironment of a freshly disaggregated and analyzed sample. Cohorts of C57BL/6 mice bearing B78 melanoma tumors were evaluated using dual lymphoid and myeloid immunophenotyping panels involving fixation and cryopreservation at three distinct points during the workflow. Results demonstrate that freezing samples after all staining and fixation are completed most accurately matches the results from noncryopreserved equivalent samples. We observed that cryopreservation of living, unfixed cells introduces a nonuniform alteration to PD1 expression. We confirm the utility of our cryopreservation protocol by comparing tumors treated with in situ tumor vaccines, analyzing both fresh and cryopreserved tumor samples with similar results. Last, we use this cryopreservation protocol with radioactive specimens to demonstrate potentially beneficial effector cell changes to the tumor immune microenvironment following administration of a novel MTRT in a dose- and time-dependent manner.

Increased indoleamine 2,3-dioxygenase activity and expression in prostate cancer following targeted immunotherapy.

BACKGROUND: We previously found that PD-L1 expression is increased on tumor cells following vaccination treatments that lead to increased tumor-specific T cells that secrete IFNγ. Indoleamine 2,3-dioxygenase (IDO) is another IFNγ inducible gene that has potent immunosuppressive effects. There have been reports of IDO expression in prostate cancer; however, it is unknown whether IDO expression might similarly increase in prostate tumors following T-cell-based immunotherapy. METHODS: Blood samples from normal male blood donors (n = 12) and patients with different stages of prostate cancer (n = 89), including patients with metastatic, castration-resistant prostate cancer treated with a DNA vaccine and/or pembrolizumab, were evaluated for IDO activity by kynurenine and tryptophan levels. Metastatic tissue biopsies obtained pre- and post-treatments were evaluated for IDO expression. IDO suppression of vaccine-induced T-cell function was assessed by ELISPOT. RESULTS: Overall, IDO activity was increased in patients with more advanced prostate cancer. This activity, and IDO expression as detected immunohistochemically, increased following treatment with either a DNA vaccine encoding the prostatic acid phosphatase (PAP) tumor antigen or PD-1 blockade with pembrolizumab. Increased IDO activity after treatment was associated with the absence of clinical effect, as assessed by lack of PSA decline following treatment. Increased antigen-specific T-cell response, as measured by IFNγ release, to the vaccine target antigen was detected following in vitro stimulation of peripheral blood cells with 1-methyltryptophan. CONCLUSIONS: These findings suggest that IDO expression is a mechanism of immune evasion used by prostate cancer and that future clinical trials using T-cell-based immune strategies might best include IDO inhibition.