Live Cell Imaging and Analysis by Sartorius 

Anton van Leeuwenhoek first observed living cells with a basic microscope in 1674.  Since then, advancements have been made in modern cell imaging techniques to resolve greater spatial and temporal resolution within cells, such as higher magnification or high-frame rate image capture to address “fast” biology.  ,    Consideration of technological advances at lower spatial or temporal detail might seem unnecessary to some. However, this would fail to recognize some key unmet user needs:
   

1. Many important biological changes occur over far longer time periods than current imaging solutions enable. For example, maturation and differentiation of stem cells can take hours, days and sometime weeks, which is hard to track using existing methods. 
2. Imaging techniques are not readily accessible to all researchers either due to expensive instrumentation or by complex software that requires a power user to use. 
3. The throughput of current solutions is typically too low for frontline use in industrial applications. 
4. Perturbance of cells in the process of imaging (e.g., fixing, loss of environmental control) can introduce unwanted and misleading experimental artifacts. 

To address these needs, a new generation of specialized compact microscopes and live-cell imaging devices have emerged. These systems are designed to reside within the controlled, stable environment of a cell incubator and gather cell images (phase contrast, bright-field and/or fluorescence) from assay microplates automatically, repeatedly and around the clock. Image acquisition is non-invasive and non-perturbing to cells, opening the opportunity to capture the full, and as needed, long-term time-course of the biology. Data is analyzed on the fly, image-by-image, to provide real-time insight into cell behavior. We refer to this paradigm, differentiated from straight live-cell imaging by the provision of analyzed data at scale, as “real-time, live-cell analysis.”

Ideally images acquired from a live-cell imaging device would be collected only from photons produced by the sample of interest, and in perfect focus.   However, corrections are usually needed due to systematic aberrations in an imaging system stemming from multiple sources such as:
   

1. Detector anomalies (e.g., detector bias, dark current variability, field flatness and thermal or gamma-ray noise) 
2. Optical issues (non-flat optical components and illumination imperfections) or undesired signal introduced by the sample
3. Autofluorescence from cellular components or media, or nonbiological signal sources such as shading, or patterns arising from sample matrices or non-uniform illumination due to meniscus effects in microwells

To perform these corrections, one must be aware of the effects of each process. Manipulations on raw images must be repeatable to ensure true capture of the measured biological signal across images, experiments and devices.  Systems that perform these corrections must provide consistency and ease of use, particularly when coupled with standardized assays, reagents and consumables which normalize the experimental process (e.g., the Incucyte^® Live-Cell Analysis System, and the assays and reagents available from Sartorius).

The consistency with which images are acquired and processed strongly influences the ability to analyze collected data. Purpose-built software that presents only the tools necessary for a specific scientific question can accelerate what might be a time-consuming step in the image analysis workflow.

While traditional compact microscopes typically only image from a single microplate or flask at a time, live-cell imaging and analysis devices such as Incucyte^®  automatically capture and analyze images from multiple microplates in parallel, thereby significantly increasing throughput (e.g., Incucyte^® = 6 x 384 well plates). With the Incucyte^® Live-Cell Analysis System, a unique moving optical path design means that the cells and cell plates remain stationary throughout the entire experiment, minimizing cell perturbance and enabling imaging and analysis of both adherent and non-adherent cell types.

Real-time, live-cell imaging and analysis has now been applied to a wide range of phenotypic cellular assays, in both two- and three-dimensional models, including:
   

• Cell proliferation
• Cell death and apoptosis
• Immune-cell killing
• Migration
• Chemotaxis
• Angiogenesis
• Neurite outgrowth and 
• Phagocytosis

The full time-course data and ‘mini-movies’ of the assay provide greater biological insight than end-point assays. Novel analyses such as area under curve, time to signal onset or threshold, and rate parameters (dx/dt) are at times highly value adding. Transient effects of treatments can be detected by kinetic imaging that may otherwise be missed with endpoint reads.

Due to its non-invasive nature, measurements from cells can be made not only during the assay itself but also during the cell preparation and ‘pre-assay’ stage. For example, the morphology and proliferation rates of cells can be monitored throughout the cell culture period and immediately post-seeding on the microplate to determine baseline measurements or observations prior to treatment. Quality control of cells and assay plates in this way helps improve assay performance and consistency by ensuring that experiments are only conducted on healthy, evenly plated cultures with the expected cell morphology.

The real-time, live-cell imaging and analysis approach also provides the opportunity to make data driven decisions while the experiment is in progress. For example, drug washout studies may be performed using real-time data to identify when an equilibrium response occurs and to trigger the timing of the washout regime. If for any reason it transpires that the experiment is not performing as expected, then treatments could be withheld to save expensive reagents and follow-on experiments can be initiated more quickly to make up time.

Real-time, live-cell imaging and analysis is extremely helpful when developing, validating, and troubleshooting phenotypic assays. Users are able to obtain a clear understanding of the relationship over time between assay signal and treatments, cell plating densities, plate coatings and other protocol parameters. Scrutiny of the kinetic data images from each well help to rapidly pinpoint sources of variance and to validate the biology of interest. This is particularly true for more advanced cell systems such as co-cultures where far more permutations and combinations of protocol parameters exist (e.g., cell plating ratios) and the biology is more complex.

Real-time, live-cell imaging and analysis is redefining the possibilities and workflows of cell biology. The combination of ease of use, throughput, long term stability and non-invasive measurement enables researchers to monitor and measure cell behaviors at a scale and in ways that were previously not possible, or at least, highly impractical.

The continued development of imaging techniques, utilization of artificial intelligence to assist in image analysis and utilization of patient-derived, complex cell models will continue to lead to scientific insight and the development of novel treatments. We have come a long way from the first recording of microorganisms with hand-drawn images, to a time where we can capture spatiotemporal events in real time.

Fixed cells have been preserved by a fixation step that “locks” the cell in place. They provide a static snapshot into cellular function. While cellular composition is compromised, larger structures such as proteins, organelles and DNA are conserved. The fixation process kills the cells, which makes the outside cell membrane more permeable than living cells. As such, it becomes easier to target internal cellular structures with dyes and antibodies. Imaging fixed cells allows high-resolution imaging of sub-cellular structures. 

However, fixed (dead) cells cannot provide dynamic insights into biological function and do not represent living systems. Live-cell imaging occurs while cells are alive. If imaged over time, real-time dynamic data can be collected and events that could be missed with endpoint, fixed cell assays are captured.

The importance in live-cell imaging and analysis lies in the ability to resolve both spatial (through higher resolution) and temporal (through time-lapse imaging) information in cells. Live-cell imaging and analysis provides dynamic insights into the health, morphology, movement and function of cell models. With live-cell imaging and analysis, we can monitor changes in real time, which is not possible with typical end-point assays that provide only a snapshot into cellular function. This is important in applications such as immuno-oncology where it becomes possible to see the real-time efficacy of cancer drugs targeting tumor cells, or in a cell migration assay where we can see how migration of tumor cells may be restricted in the presence of cancer drugs. 

Live-cell imaging and analysis also provides the opportunity to make data driven decisions while the experiment is in progress. A researcher studying the biology of vascular or neuronal networks, for example, may wish to first establish a stable network before assessing the effects of compound treatments or genetic manipulations (e.g. siRNAs). With continuous live-cell imaging and analysis, it is straightforward to temporally track network parameters and use the real-time data to judge when best to initiate treatment regimes. The timing of adjunct studies such as analysis of metabolites or secreted proteins in supernatants can also be guided.

Live-cell imaging and analysis should be used when studying any area of cell therapeutics. Traditional end-point assays only provide single measurement of cellular events. Cells are dynamic in nature, so it is important to have the ability to image in real time to gain access to deeper biological insights. Live-cell imaging and analysis is typically used for therapeutic areas such as immunology, oncology, immuno-oncology and neuroscience to study cell health and proliferation, cell movement and morphology and cell function.

Images are typically visualized using microscopy techniques. Optical microscopes are a standard tool used in most labs and can visualize structures up to ~200nm range. With advancements in modern high-resolution imaging techniques and the synthesis of fluorescent probes, it is now possible to view labeled sub-cellular structures at the 10-50 nm scale. With increasing technical advancements comes increased complexity and costs; high-resolution microscopes often require extensive training and access is often limited. 

High-content screening and analysis systems are benchtop systems that combine fluorescent microscopy with analysis software to visualize and analyze real-time cellular data. These systems typically do not sit within a tissue culture incubator and so are limited in terms of environmental stability and long-term studies. 

Multi-mode readers are microplate readers that can detect two or more of the following: luminescence, fluorescence, time-resolved fluorescence and absorbance. An imaging system within the multi-mode reader can also image wells during the assay. These systems again lack proper environmental controls which will impact long-term studies. 

Live-cell imaging and analysis systems are designed to capture cellular events as they occur in real time. Researchers can analyze a series of data-points over time, rather than a single time-point that does not provide the full picture of what their cells are doing.  

Live-cell imaging and analysis systems housed within an incubator, such as the Incucyte^®, can perform real-time continuous analysis over days to months, while keeping cells in a stable and optimal environment. Many technologies such as multi-mode readers and high-content imagers do not have the ability to maintain environmental control, meaning cells are not kept at physiologically relevant conditions. 

Depending on application needs, the Incucyte^® is also capable of running up to six microplates in parallel, allowing more throughput than typical microscopy and single plate readers. In addition to this, the Incucyte^® is designed to be user-friendly. Reagents have been optimized for each application allowing easy assay preparation and intuitive software that guides the user through each step.