## 4 Discussion  
Monocytes are critical for the defense against microbial infections, but also for promoting resolution of inflammation. However, an improper balance of these tasks can lead to the collateral damage of host tissues and delay of tissue regeneration [42]. The rationale for the present study arose from the striking differences in the function of classical proinflammatoryamy \(\mathrm{Ly6C^{hi}}\) monocytes revealed by \(Ccr2^{/-/}\). Whereas the lack in the egress from the bone marrow and hence the recruitment of \(\mathrm{Ly6C^{hi}}\) monocyte prevented liver destruction after \(E\). _histolytica_ infection [16], their absence in \(L\). _monocytogenes_ infection  
Figure 6: CD38\({}^{+}\)Ly6C\({}^{hi}\) monocytes represent the major monocytic source of ROS production and contribute to liver damage following \(E\). _histolytica_ infection. (**A**) Heat map of differentially regulated genes involved in ROS production and a graph showing fold changes in expression of _Nof1_ mRNA in the liver during infection with \(E\). _histolytica_ or \(L\). _monocytogenes_. (**B**) Gating scheme and histogram for ROS\({}^{+}\) liver \(\mathrm{Ly6C^{hi}}\) monocytes. Percentage and mean fluorescent intensity (MFI) of ROS\({}^{+}\)Ly6C\({}^{hi}\) monocytes during (**C**) _E. _histolytica_ and (**D**) _L. _monocytogenes_ infection. (**E**) Percentage of ROS\({}^{+}\)CD38\({}^{+}\) out of \(\mathrm{Ly6C^{hi}CD11b^{+}}\) monocytes in both infection models. (**F**) Percentage of ample liver abscess weight in relation to the left liver lobe in WT (C57 BL/6) and _Cd38\({}^{-}\)_- mice. (**G**) Percentage of \(\mathrm{Ly6C^{hi}}\) CD11b\({}^{+}\) cells in the liver in naive, infected WT and _Cd38\({}^{-}\)_- mice. (* \(p<0.05\); ** \(p<0.01\); *** \(p<0.001\); *** \(p<0.0001\); Mann-Whitney U test). exacerbated disease progression [23], pointing to functional differences in this monocyte subset which was originally regarded as homogeneous. Recent studies based on single-cell sequencing actually suggest an even greater diversity. Under conditions of homeostasis three more proinflammatory monocyte subsets: nMO, dcdMO [12; 13; 43; 44], and _Cxcl10\({}^{+}\)_ and _Saa3\({}^{+}\)_ monocytes were identified [14]. Interestingly, all analyzed genes involved in development of nMO were strongly upregulated in proinflammatory monocytes during _L. monocytogenes_ infection, but they were unaffected in monocytes from the amebic model. Enhanced development of Ly6C\({}^{\rm hi}\) monocytes into nMO has been demonstrated previously, but only under LPS stimulation [45] and the present study is the first to demonstrate its presence in vivo by bacterial infection. Likewise, genes related to MHC-II-mediated antigen presentation and dcdMO development were upregulated in listeria infection. However, some factors thought to be important for dcdMO development (i.e., _Flt3_, _Pou2f2_, _Pid1_, and _Hpgd_) were strongly downregulated while their expression by monocytes from the amebiasis model was comparably higher. In addition, genes characteristic for the subset of _Cxcl10\({}^{+}\)_ and _Saa3\({}^{+}\)_ monocytes that arise under sterile inflammatory conditions (e.g., autoimmune encephalitis) [14] were only upregulated in the listeria model. Taken together, the data suggest that proinflammatory monocytes from _L. monocytogenes_-infected animals display a distinct proinflammatory phenotype, characterized by the upregulation of genes associated with nMO, dcdMO and _Cxcl10\({}^{+}\)_ and _Saa3\({}^{+}\)_ cells. Additional relevant transcriptional differences between Ly6C\({}^{\rm hi}\) monocytes from both models became apparent by examining the expression of selected transcription factors and genes involved in polarization of monocytes. Ly6C\({}^{\rm hi}\) monocytes from the parasite model exhibited a more anti-inflammatory phenotype and the expression of genes involved in conversion of Ly6C\({}^{\rm hi}\) monocytes to Ly6C\({}^{\rm lo}\) monocytes.