Biological research increasingly demands the use of non-invasive and ultra-sensitive imaging techniques. The Airyscan technology was recently developed to bridge the gap between conventional confocal and super-resolution microscopy. This technique combines confocal imaging with a 0.2 Airy Unit pinhole, deconvolution and the pixel-reassignment principle in order to enhance both the spatial resolution and signal-to-noise-ratio without increasing the excitation power and acquisition time. Here, we present a detailed study evaluating the performance of Airyscan as compared to confocal microscopy by imaging a variety of reference samples and biological specimens with different acquisition and processing parameters. We found that the processed Airyscan images at default deconvolution settings have a spatial resolution similar to that of conventional confocal imaging with a pinhole setting of 0.2 Airy Units, but with a significantly improved signal-to-noise-ratio. Further gains in the spatial resolution could be achieved by the use of enhanced deconvolution filter settings, but at a steady loss in the signal-to-noise ratio, which at more extreme settings resulted in significant data loss and image distortion.

Quantifying small, rapidly evolving forces generated by cells is a major challenge for the understanding of biomechanics and mechanobiology in health and disease. Traction force microscopy remains one of the most broadly applied force probing technologies but typically restricts itself to slow events over seconds and micron-scale displacements. Here, we improve >2-fold spatially and >10-fold temporally the resolution of planar cellular force probing compared to its related conventional modalities by combining fast two-dimensional total internal reflection fluorescence super-resolution structured illumination microscopy and traction force microscopy. This live-cell 2D TIRF-SIM-TFM methodology offers a combination of spatio-temporal resolution enhancement relevant to forces on the nano- and sub-second scales, opening up new aspects of mechanobiology to analysis.

Quantifying small, rapidly progressing three-dimensional forces generated by cells remains a major challenge towards a more complete understanding of mechanobiology. Traction force microscopy is one of the most broadly applied force probing technologies but ascertaining three-dimensional information typically necessitates slow, multi-frame z-stack acquisition with limited sensitivity. Here, by performing traction force microscopy using fast single-frame astigmatic imaging coupled with total internal reflection fluorescence microscopy we improve the temporal resolution of three-dimensional mechanical force quantification up to 10-fold compared to its related super-resolution modalities. 2.5D astigmatic traction force microscopy (aTFM) thus enables live-cell force measurements approaching physiological sensitivity.

The CD2 receptor has been described as an adhesion and costimulatory receptor on T cells. Here, transcriptional profiling of colorectal cancers (CRC) revealed a negative correlation between CD2 expression and “exhausted CD8 ⁺ T-cells” gene signatures. Furthermore, we detected reduced surface CD2 levels in exhausted CD127 ^low PD-1 ^hi CD3 ⁺ CD8 ⁺ tumour infiltrating lymphocytes (TILs) in CRC. We describe a CD2 expression-level-dependent switch in CD2-CD58 localization between central and peripheral domains in the immunological synapse (IS). A peripheral “CD2 corolla” formed when CD2 surface expression was sufficiently high and its cytoplasmic domain intact. The corolla recruited other ligated receptors like CD28, boosted recruitment of activated Src-family kinases (pSrc), LAT and PLC-γ in the IS and consequently T-cell activation in response to a tumour antigen. Corolla formation and pSrc in the IS increased linearly with CD2 expression, whereas pSrc signals were reduced by high, “exhausted-like” levels of PD-1, which invaded the corolla. These results suggest two levels of inhibition of Src-family kinases in CD3 ⁺ CD8 ⁺ TILs: reduced CD2 expression and high PD-1 expression.

Quantifying cell generated mechanical forces is key to furthering our understanding of mechanobiology. Traction force microscopy (TFM) is one of the most broadly applied force probing technologies, but its sensitivity is strictly dependent on the spatio-temporal resolution of the underlying imaging system. In previous works, it was demonstrated that increased sampling densities of cell derived forces permitted by super-resolution fluorescence imaging enhanced the sensitivity of the TFM method. However, these recent advances to TFM based on super-resolution techniques were limited to slow acquisition speeds and high illumination powers. Here, we present three novel TFM approaches that, in combination with total internal reflection, structured illumination microscopy and astigmatism, improve the spatial and temporal performance in either two-dimensional or three-dimensional mechanical force quantification, while maintaining low illumination powers. These three techniques can be straightforwardly implemented on a single optical set-up offering a powerful platform to provide new insights into the physiological force generation in a wide range of biological studies. This article is part of the Theo Murphy meeting issue ‘Super-resolution structured illumination microscopy (part 1)'.

Background: Metabolic dysfunction-associated steatotic liver disease is defined by hepatic lipid overload resulting in a metabolic shift and subsequent mitochondrial impairment. Diagnosis currently relies on tissue biopsy and non-invasive tests. However, these have drawbacks, including subjective histology scoring and relatively low sensitivity, highlighting the need for more robust and reproducible methodologies. Methods: Fluorescence lifetime imaging microscopy visualises the metabolic state of cells by measuring the autofluorescence lifetime of metabolites, effectively avoiding the need for exogenous labelling. This technique was applied to a broad range of models, spanning from a hepatocyte cell line to a human tissue slice model, to investigate metabolic changes across disease conditions. Results: Here, by utilising the metabolic dysfunction associated with steatotic liver disease, we propose a time-efficient method and introduce an index as a quantitative output to assess the metabolic state of human liver biopsies. The index encapsulates features of metabolic dysfunction that directly report on the disease state. These findings using lifetime imaging are substantiated by extensive analysis of structural and functional mitochondrial dysfunction. Conclusions: Measuring fluorescence lifetime can capture features of metabolic change that standard histological methods do not. Correlating the results to established techniques of histological evaluation highlights the potential of this method to enhance characterisation and speed of biopsy results in metabolically implicated diseases.