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Immune response dynamics in COVID-19 and its severe manifestations have largely been studied in circulation. Here, we examined the relationship between immune processes in the respiratory tract and circulation through longitudinal phenotypic, transcriptomic and cytokine profiling of paired airway and blood samples from patients with severe COVID-19 relative to heathy controls. In COVID-19 airways, T cells exhibited activated, tissue-resident, and protective profiles; higher T cell frequencies correlated with survival and younger age. Myeloid cells in COVID-19 airways featured hyper-inflammatory signatures and higher frequencies of these cells correlated with mortality and older age. In COVID-19 blood, aberrant CD163+ monocytes predominated over conventional monocytes, and were found in corresponding airway samples and in damaged alveoli. High levels of myeloid chemoattractants in airways suggest recruitment of these cells through a CCL2-CCR2 chemokine axis. Our findings provide insights into immune processes driving COVID-19 lung pathology with therapeutic implications for targeting inflammation in the respiratory tract., Through longitudinal profiling of paired airway and blood from patients with severe COVID-19, Szabo et al. reveal airway immune responses that correlate with age and outcome. They further identify coordinate roles for T and myeloid cells in the respiratory tract and circulation in perpetuating lung pathology and disease pathogenesis.

Clear Cell Renal Carcinoma (ccRCC) is a heterogeneous disease with a variable post-surgical course. To assemble a comprehensive ccRCC tumor microenvironment (TME) atlas, we performed single-cell RNA sequencing (scRNASeq) of hematopoietic and non-hematopoietic subpopulations from tumor and tumor-adjacent tissue of treatment-naïve ccRCC resections. We leveraged the VIPER algorithm to quantitate single-cell protein activity, and validated this approach by comparison to flow cytometry. The analysis identified key TME sub-populations, as well as their master regulators and candidate cell-cell interactions, revealing clinically-relevant populations, undetectable by gene expression analysis. Specifically, we uncovered a tumor-specific macrophage subpopulation characterized by upregulation of TREM2/APOE/C1Q - validated by spatially-resolved, quantitative multispectral immunofluorescence. In a large clinical validation cohort, these markers were significantly enriched in tumors from patients who recurred following surgery. The study thus identifies TREM2/APOE/C1Q-positive macrophage infiltration as a potential prognostic biomarker for ccRCC recurrence, as well as a candidate therapeutic target.

Primary Clear Cell Renal Carcinoma (ccRCC) is a highly heterogenous disease with a variable disease course post-surgery, and ccRCC tumor micro-environment has thus far not been characterized at the single-cell level with the whole-transcriptome resolution enabled by single-cell RNA Sequencing (scRNASeq). To elucidate the cellular and transcriptional mechanisms driving disease recurrence, we performed scRNASeq on both hematopoietic and non-hematopoietic populations from tumor and tumor-adjacent tissue from primary resections of treatment-naïve ccRCC patients (n=11), thus producing a complete atlas of the ccRCC tumor microenvironment. Furthermore, in order to mitigate gene dropout inherent to scRNASeq and commonly preventing detection of >80% of genes, we leveraged the VIPER algorithm, which quantitates protein activity, to infer the proteomic regulators of cell state. Clustering at the gene expression level enabled construction of lineage-specific gene regulatory networks applying ARACNe, the Algorithm for Reconstruction of Accurate Cellular Networks, from which protein activity of upstream regulatory molecules could be inferred by VIPER. Comparison with protein-level expression data from antibody-based, high-parameter spectral flow cytometry in the same patients shows that VIPER-measured protein activity systematically abrogated gene dropout effects on a repertoire of >6,000 regulatory proteins, comparing favorably with antibody-based measurements. This helped comprehensively characterize the individual cellular sub-populations comprising the ccRCC tumor and peri-tumor microenvironment, as well as their specific master regulators and candidate cell-cell interactions, revealing several populations undetectable by gene expression analysis. Specifically, we uncovered a novel tumor-specific, macrophage subpopulation characterized by significant upregulation of TREM2, APOE, and C1Q, as validated by spatially-resolved, quantitative multispectral immunofluorescence (qmIF). These tumor-specific macrophages were statistically significantly over-represented in a separate validation cohort of patients who recurred following surgery (n=4) compared to patients who did not recur (n=4). This study highlights the substantial resolution increase afforded by VIPER and identifies TREM2/APOE/C1Q-positive macrophage infiltration as a potential biomarker for ccRCC recurrence and a candidate target for intervention. Furthermore, it provides a highly-generalizable methodology to study the role of rare subpopulations by leveraging VIPER on scRNASeq data. Citation Format: Aleksandar Obradovic, Nivedita Showdhury, Casey Ager, Vinson Wang, Lukas Vlahos, Xinzheng V. Guo, David H. Aggen, James McKiernan, Andrea Califano, Charles G. Drake. Tumor-specific cell populations in clear cell renal carcinoma associated with clinical outcome identified using single-cell protein activity inference [abstract]. In: Proceedings of the AACR Virtual Special Conference on Tumor Heterogeneity: From Single Cells to Clinical Impact; 2020 Sep 17-18. Philadelphia (PA): AACR; Cancer Res 2020;80(21 Suppl):Abstract nr PO-024.

Large-scale immune monitoring experiments (such as clinical trials) are a promising direction for biomarker discovery and responder stratification in immunotherapy. Mass cytometry is one of the tools in the immune monitoring arsenal. We propose a standardized workflow for the acquisition and analysis of large-scale mass cytometry experiments. The workflow includes two-tiered barcoding, a broad lyophilized panel, and the incorporation of a fully automated, cloud-based analysis platform. We applied the workflow to a large antibody staining screen using the LEGENDScreen kit, resulting in single-cell data for 350 antibodies over 71 profiling subsets. The screen recapitulates many known trends in the immune system and reveals potential markers for delineating MAIT cells. Additionally, we examine the effect of fixation on staining intensity and identify several markers where fixation leads to either gain or loss of signal. The standardized workflow can be seamlessly integrated into existing trials. Finally, the antibody staining data set is available as an online resource for researchers who are designing mass cytometry experiments in suspension and tissue.

High dimensional cytometry now allows measurement of over 50 parameters in a single sample, and is typically visualized using sophisticated dimensionality-reducing methods and analyzed with automated clustering algorithms. While these tools facilitate the identification and presentation of key findings, it remains challenging to effectively monitor and report the staining quality of individual markers. We present the Average Overlap Frequency (AOF), a simple and efficient metric to evaluate and quantify the robustness of staining and clustering quality in high-dimensional data. We leverage the AOF to compare and determine the optimal storage conditions for stained whole blood samples prior to mass cytometry analysis. We also show that the AOF can be easily incorporated as part of automated analysis pipelines in large scale immune monitoring studies and used to flag and exclude samples with poor staining quality. We propose that the AOF may be incorporated as an essential quality control metric to better identify and report the underlying sample quality in all CyTOF and other high-dimensional cytometry experiments.

Summary Human intestinal transplantation often results in long-term mixed chimerism of donor and recipient blood in transplant patients. We followed the phenotypes of chimeric peripheral blood cells in 21 patients receiving intestinal allografts over 5 years. Donor lymphocyte phenotypes suggested a contribution of hematopoietic stem and progenitor cells (HSPCs) from the graft. Surprisingly, we detected donor-derived HSPCs in intestinal mucosa, Peyer's patches, mesenteric lymph nodes, and liver. Human gut HSPCs are phenotypically similar to bone marrow HSPCs and have multilineage differentiation potential in vitro and in vivo. Analysis of circulating post-transplant donor T cells suggests that they undergo selection in recipient lymphoid organs to acquire immune tolerance. Our longitudinal study of human HSPCs carried in intestinal allografts demonstrates their turnover kinetics and gradual replacement of donor-derived HSPCs from a circulating pool. Thus, we have demonstrated the existence of functioning HSPCs in human intestines with implications for promoting tolerance in transplant recipients.

Many processes that are essential for mycobacterial growth are poorly understood. To facilitate genetic analyses of such processes in mycobacteria, we and others have developed regulated expression systems that are repressed by a tetracycline repressor (TetR) and induced with tetracyclines, permitting the construction of conditional mutants of essential genes. A disadvantage of these systems is that tetracyclines function as transcriptional inducers and have to be removed to initiate gene silencing. Recently, reverse TetR mutants were identified that require tetracyclines as corepressors. Here, we report that one of these mutants, TetR r1.7, allows efficient repression of lacZ expression in Mycobacterium smegmatis in the presence but not the absence of anhydrotetracycline (atc). TetR and TetR r1.7 also allowed efficient silencing of the essential secA1 gene, as demonstrated by inhibition of the growth of a conditional mutant and dose-dependent depletion of the SecA1 protein after the removal or addition, respectively, of atc. The kinetics of SecA1 depletion were similar with TetR and TetR r1.7. To test whether silencing of secA1 could help identify substrates of the general secretion pathway, we analyzed the main porin of M. smegmatis , MspA. This showed that the amount of cell envelope-associated MspA decreased more than 90-fold after secA1 silencing. We thus demonstrated that TetR r1.7 allows the construction of conditional mycobacterial mutants in which the expression of an essential gene can be efficiently silenced by the addition of atc and that gene silencing permits the identification of candidate substrates of mycobacterial secretion systems.

The identification of Mycobacterium tuberculosis genes necessary for persistence in vivo provides insight into bacterial biology as well as host defense strategies. We show that disruption of M. tuberculosis membrane protein PerM (Rv0955) resulted in an IFN-γ-dependent persistence defect in chronic mouse infection despite the mutant’s near normal growth during acute infection. The perM mutant required increased magnesium for replication and survival; incubation in low magnesium media resulted in cell elongation and lysis. Transcriptome analysis of the perM mutant grown in reduced magnesium revealed upregulation of cell division and cell wall biosynthesis genes, and live cell imaging showed PerM accumulation at the division septa in M. smegmatis. The mutant was acutely sensitive to β-lactam antibiotics, including specific inhibitors of cell division-associated peptidoglycan transpeptidase FtsI. Together, these data implicate PerM as a novel player in mycobacterial cell division and pathogenesis, and are consistent with the hypothesis that immune activation deprives M. tuberculosis of magnesium., Author Summary The success of Mycobacterium tuberculosis (Mtb) as a human pathogen is due to ability to persist in chronic infection, despite a robust adaptive immune response by the host. The mechanisms by which Mtb achieves this are, however, poorly understood. Here we show that a novel integral membrane protein, Rv0955/PerM, is essential for Mtb persistence during chronic mouse infection. The perM mutant required increased magnesium compared to wild type Mtb for replication and survival in culture and elongated in media with reduced magnesium concentration. Transcriptomic, electron microscopy and live cell imaging approaches provided evidence that PerM is involved in cell division. The survival defects of the perM mutant in reduced magnesium and during chronic mouse infection are consistent with the hypothesis that magnesium deprivation constitutes an IFN-γ dependent host defense strategy. This work also has potential clinical implications, as disruption of PerM renders Mtb susceptible to β-lactam antibiotics, which are commonly used to treat non-mycobacterial infections.

Gene expression systems that allow the regulation of bacterial genes during an infection are valuable molecular tools but are lacking for mycobacterial pathogens. We report the development of mycobacterial gene regulation systems that allow controlling gene expression in fast and slow-growing mycobacteria, including Mycobacterium tuberculosis, using anhydrotetracycline (ATc) as inducer. The systems are based on the Escherichia coli Tn10-derived tet regulatory system and consist of a strong tet operator (tetO)-containing mycobacterial promoter, expression cassettes for the repressor TetR and the chemical inducer ATc. These systems allow gene regulation over two orders of magnitude in Mycobacterium smegmatis and M.tuberculosis. TetR-controlled gene expression was inducer concentration-dependent and maximal with ATc concentrations at least 10- and 20-fold below the minimal inhibitory concentration for M.smegmatis and M.tuberculosis, respectively. Using the essential mycobacterial gene ftsZ, we showed that these expression systems can be used to construct conditional knockouts and to analyze the function of essential mycobacterial genes. Finally, we demonstrated that these systems allow gene regulation in M.tuberculosis within the macrophage phagosome.