Artificial intelligence applications are gaining popularity in pathology because they bear the attractive potential to overwhelm the rate-limiting step of human processing: the low throughput. Deep learning-based immunopathology takes advantage of high-throughput data from digital pathology
[38][39][40][41]. Both supervised and unsupervised machine learning applications are implemented in immunoneuropathology for different purposes
[42] and claim improvements in the reading time as well as solutions to subjectivity problems such as the inter-reader variability.
Cell classification from peripheral blood samples is an established computational field with many competing applications. Kutlu et al. compared recurrent convolutional neuronal networks for the white blood cell classification from peripheral blood samples, finding a superior performance of Res-Net
[43]. Beyond peripheral blood samples, deep learning algorithms can cope with the complexity of tissues and challenge not only the manual histological semiquantitative imaging, but also older methods of unbiased cell counting
[44].
3.1. Microglial Segmentation and Counting
In contrast to peripheral blood cells, MG should be segmented from a noisy background
[45]. The highly ramified MG structure
[29], described as
lacunarity and fractal dimension in the technical language of applied mathematics and image analysis, is at the same time a good indicator of the microglial activation status and a technical challenge for computer vision in immunopathology
[46][47]. Previous attempts to automatically segment MG have reached a detection accuracy from 80–90%, facing, however, the problem of false negatives due to cell overlapping, texture variabilities, noisy background, and staining inhomogeneities that prohibit the success of standard thresholding models
[45][48][49].
Liu et al. circumvented the bottleneck of manual histological image analysis of arborized cells by an unsupervised machine learning pipeline for the high throughput counting of MG and astrocytes
[50]. The pipeline improved the analysis time 200 times using an Opera Phenix High Content Screening (PerkinElmer Inc., Hamburg, Germany) high-throughput imaging input. PhenoLOGIC (Phenologic, MI, USA) computer vision is a training image set utilizing supervised machine learning to differentiate background from tissue, integrated by the Harmony High Content Imaging and Analysis software (PerkinElmer), which segmented MG by intensity thresholding. The suggested pipeline for automated image analysis provides the total
Iba1 brain coverage and extends the analysis to other cellular compartments, such as astrocytes (GFAP and AQP4). Most false-negative results were derived due to inhomogeneities in staining intensity, thus confirming previous observations from independent groups that staining intensity fluctuations are a significant burden for artificial intelligence methods in cell counting
[45][49].
Unbiased stereology is an established statistical model for predicting the density of geometrical structures in space (e.g., cells, cellular processes or arbors, synapses, particles) based on randomized cell counting from thick tissue slices. Stereology has contributed to significant advances in the field of neuroimmunology, bridging immune reactions with cognition
[51], dementia
[52], and epilepsy
[53], among others. Mouton et al.
[54] performed a longitudinal scientific work towards the automation of the software
Stereologer® (SRC Biosciences, Tampa FL, USA), and suggested an
Automated Segmentation Algorithm (ASA) for the deep learning stereology of immunostained neurons and MG in mouse neocortex. ASA is intended to work in a human-in-the-loop interactive pipeline to perform cell segmentation without a priori shape assumptions. Despite the increased shape-complexity of MG, ASA performed a better detection of MG than NeuN-stained neurons
[54]. The primary software drawback was the spatial cell overlapping. ASA was subjected to improvements by the same group; Alahmari et al. introduced a next-generation unbiased stereology approach, the
FAST-Stereology (Fully Automated Stereology Technology), in a model that improves ASA reading time and reliability for neuronal detection with less than 2% error
[55]. The deep learning convolutional neuronal network U-net
[56] is trained on a supervised mask for automated cell counts in the dissector field, thus boosting unbiased stereology with the multiplication power of deep learning. FAST was successfully tested for NeuN measurements in mouse neocortex slices and opened new frontiers for the measurement of more complex and challenging cell appearances, such as densely packed, ramified cells and fragmented branches of astroglia and MG.
Horvath et al. brought about improvements in the detection of ramified cells, influenced by the innovative work of Suleymanova et al.
[57]. A deep convolutional neural network approach for astrocyte detection (made available in the software platform FindMyCells©,
www.findmycells.org, accessed on 1 July 2021) outperformed classical methods such as ilastik© (
https://www.ilastik.org/, accessed on 1 July 2021) and ImageJ© (
https://imagej.nih.gov/ij/, accessed on 1 July 2021) in both accuracy and time performance. Compared to manual counts, FindMyCells did not underperform human intelligence in astrocyte counting. Challenging FindMyCells with MG counts and one-by-one by comparing FindMyCells with ASA- or FAST Stereology are open challenges in the field of automated quantitative neuroimmunopathology.
Beyond the field of cell detection and segmentation in statical images, immunoinformatics shapes the field of cell detection and cell tracking in video microscopy. Gregorio da Silva et al.
[58] trained a network to detect leukocyte recruitment using
intravital video microscopy in different contexts, including inflammatory models of the CNS. By setting their available code, the authors provide a valuable immunoinformatic tool to the field of inflammation research in models such as the experimental autoimmune encephalomyelitis
[59].
3.2. Cell Arborization Analysis
Dendritic arborization, also known as dendritic branching, is the property of MG, neurons, astrocytes, and other cells, to form new dendritic trees and branches, which anatomically support the establishment of new contact points to their environment. While manual reconstructions of the cell bodies and arbors using standard tools such as Neurolucida
® (MBF Bioscience, Williston, VT, USA) can provide clues on the brain immune status
[53], they are extremely time-consuming. The urge for an errorless and time-effective automated arborization analysis of cell traces has two constituents: (i) automated cell tracing and (ii) systematic, unbiased quantitative arborization analysis.
Ascoli et al. extensively addressed the problem of a time-effective and objective arborization analysis. Introduced by Scorcioni et al. as the
LM-tool and improved by Luisi et al. in the
FARSIGHT tool, this standalone freeware platform offers a multiparametric quantitative arborization analysis using unsupervised co-clustering
[60][61]. Lu et al.
[62] developed the Scorcioni L-Measure by adding diffusion distance measure and harmonic analysis. Lu et al. identified hierarchical arborization in reconstructed cells and offered a valuable tool for creating hypothesis-free assumptions on microglia morphology and function. Available as the FARSIGHT standalone tool or packaged for MATLAB (MathWorks, Natick, MA, USA), Lu’s quantitative analysis is a powerful analysis tool, limited only by the availability of computational resources.