Fig 2: Lack of monocyte recruitment, no overt parasite killing, and low L. major proliferation in the absence of iNOS induction in Rag1-/- mice(A–D) Low-dose L. majornecR-infected WT and Rag1-/- mice analyzed 5 wpi. (A) Recruited immune cells (top), iNOS expression (middle), and pathogen burden (bottom) at the site of infection. Each circle represents one infected ear. Data pooled from 2 (flow cytometry) to 3 (limiting dilution) experiments with at least 9 ears per group. (B–D) Assessment of parasite death in WT versus Rag1-/- mice employing the L. majornecR reporter strain. (B) IV-2PM (upper panel) and corresponding heatmaps of parasite death (lower panel). Projections of at least 20 z planes spaced 3 µm apart are shown. Scale bar, 20 µm. (C) Representative large-field IV-2PM in WT and Rag1-/- mice depicting the CFP-to-mNectarine ratios of single pathogens (upper panel) and corresponding quantitative analysis (lower panels). Projections of at least 25 z planes spaced 3 µm apart are shown. Scale bar, 100 µm. (D) Fraction of dying parasites in WT versus Rag1-/- in 8 infected ears per group. Data pooled from 2 independent experiments. Horizontal bars denote the mean. ***p < 0.001, according to Mann-Whitney test.(E–G) Parasite proliferation in WT versus Rag1-/- mice employing IV-2PM of the reporter strain L. majorSWITCH. (E) IV-2PM examples of WT and Rag1-/- ears infected with L. majorSWITCH 48 h after photoconversion. Scale bar, 20 µm. Segmented and masked red and green channels, as well as the collagen SH signal (gray), are shown from one z plane of a three-dimensional image. (F) Quantifications of recovery from photoconversion (black) with measurements of the same site before (green) and 0 h after (red) photoconversion was overlaid. Data normalized as described in STAR Methods. (G) Fraction of proliferating parasites in WT versus Rag1-/- mice. Each symbol represents one imaged infection site. Data accumulated from at least 7 imaged areas acquired in 3 independent experiments. Horizontal bars represent the mean. **p < 0.001; *p < 0.05; ns, not significant, according to Mann-Whitney test in (A), (D), and (G).See also Figure S5.

Fig 3: Monocyte administration, but not neutrophil recruitment, increases pathogen burden at later stages of the infection(A) Flow cytometry analysis of high-dose infection sites injected at 10 wpi with PBS (top row) or with 2 × 106 iNOS-expressing (middle row) or non-activated (bottom row) bone marrow monocytes, analyzed 3 days after injection.(B) Analysis of pathogen burden (top panel) and infection rate in Ly6G-CD11b+ monocyte-derived cells by flow cytometry (bottom panel). Each circle in (B)–(D) represents one infected ear at 10 wpi (high-dose infection) injected with PBS (black circles) or with activated (filled red circles) or non-activated (empty red circles) bone marrow monocytes into the infection site three days before analysis.(C) Fraction of injected CD45.1+ donor cells among all Ly6G-CD11b+ monocyte-derived cells at the site of infection.(D) Total number of Ly6G-CD11b+ monocyte-derived cells (left) and Ly6G+CD11b+ neutrophils (right) isolated from the site of infection. Horizontal bars in (B)–(D) denote the median.(E) Flow cytometry of infection sites treated epicutaneously with vehicle (top panels) DNFB at 10 wpi (high-dose infection), 3 days before analysis.(F– H) Cell number (F) and fraction of infected cells (G) determined for Ly6G-CD11b+ monocyte-derived cells (left) and Ly6G+CD11b+ neutrophils (right) isolated from the site of infection, as well as pathogen burden (H). Black circles in (F)–(H) represent vehicle-treated infection sites, and red circles represent DNFB-treated infection sites. Each circle represents one infected ear. Data collected in at least two independent experiments. Horizontal bars denote the median.ns, not significant; *p < 0.05; ***p < 0.001, according to Kruskal-Wallis with Dunn’s post-test in (B)–(D), (F), and (G) or Mann-Whitney test in (H). See also Figure S7.

Fig 4: Ordinary differential equation-based modeling of L. major high- and low-dose infection(A–E) Time course of low-dose (5 × 103 metacyclics) L. major infection analyzed by limiting dilution (A), flow cytometry (B), hematoxylin and eosin staining (C), and intracellular flow cytometry staining of iNOS (D and E). ***p < 0.001; **p < 0.01; *p < 0.05; ns, not significant, according to Kruskal-Wallis with Dunn’s post-test (comparison with 1 wpi data). Scale bar, 200 µm.(F) Ordinary differential equation model of monocyte-derived cells according to their activation (iNOS expression) and infection state. Arrows: possible state transitions. M, non-activated, non-infected cells; Ma, activated (iNOS expressing); Mi, infected; Mai, activated and infected; P, parasite intracellular proliferation. Gray shading: activation (iNOS expression), red: parasites.(G) L. major containment in the different models. Blue: cell-intrinsic killing and proliferation inhibition by activated, infected cells (models 1–3). Green: cell-extrinsic killing and proliferation inhibition within all infected cells exerted by all activated cells (models 4–6).(H and I) High-dose (2 × 106 stationary parasites, H) and low-dose (5 × 103 purified metacyclic parasites, I) infection data used to inform the models. Each symbol in (A), (B), (E), (H), and (I) represents one infected ear. Horizontal bars denote the mean. Data cumulated from at least two independent experiments. Parasite burden data in (I) replotted from (A) as mean + standard deviation. Overlays of best fits for model 1 (solid lines) and model 4 (dashed lines) are shown.(J) Costs of fitting plotted against quality of the model determined as a function of the AICc (corrected Akaike information criterion). Low-cost, high-quality models appear in the lower left corner of the graphs. The AICc of the model with the lowest AICc (model 4) was set to 0. The models that cannot be excluded by e(2 ×?AICc) are represented by filled circles; all other models are represented by empty circles.See also Figure S1.

Fig 5: Blocking monocyte recruitment abolishes the iNOS inhibition-dependent increase in L. major proliferation and affects the course of infection(A) Experimental setup, high-dose infections.(B) Ear thickness at day 21.(C) Fraction of Ly6C+ cells (gated according to Figure S6) recruited within the last four days before analysis. Each dot in (A) and (B) represents an individual ear. Data pooled from two independent experiments.(D) Parasite proliferation determined by IV-2PM. Top row: IV-2PM data. Scale bar, 20 µm. Segmented red and green channels and collagen SH signals (gray) are shown from one z plane of a three-dimensional image. Bottom row: quantifications (black) of the images shown with measurements of the same site before (green) and 0 h after (red) photoconversion was overlaid. Data normalized as described in STAR Methods.(E) Fraction of proliferating parasites. Each symbol represents one imaged area. Data from at least 8 imaged areas acquired in three independent experiments.(F) Setup for transient iNOS inhibition (blue) and anti-CD18/CD49d-mediated recruitment blocking (red).(G) Simulated infection rates for models 1 and 4 (fraction of Mi + Mai among all monocytes) modeled according to (F). Curves show the time course from day 41 to day 43. Numbers indicate the model (1 or 4) for each curve.(H) Fraction of infected Ly6G-CD11b+ monocyte-derived cells determined by flow cytometry at day 42 p.i., treated according to (F). Each circle represents one infected ear.(I–K) Metabolic flux analysis of monocyte-derived cells isolated from the infected ear tissue at 3 wpi. (I) Left panel: oxygen consumption rate (OCR) in cells from control (black), L-NIL-treated (blue), L-NIL and anti-CD18/CD49d-treated (magenta), and Rag1-/- (red) animals. Right panel: basal OCR and extracellular acidification rate (ECAR). Circles show the mean ± SEM of at least 5 replicates, consisting of pools of isolated monocytes normalized to 105 viable cells, collected in three independent experiments from 16 infected ears per group. (J and K) Maximum respiration (J) and spare respiratory capacity (K) under the different conditions described under (I). Each circle shows one replicate of 105 pooled viable cells, collected in three independent experiments from 16 infected ears per group in total.(L) Quantitative PCR of pro-IL-1ß expression by monocytes as described under (I). Each circle represents one infected ear.Horizontal bars denote the median. ns, not significant; *p < 0.05; **p < 0.01; ***p < 0.001, according to Kruskal-Wallis with Dunn’s post-test (comparison with control [no treatment, Rag1+/+ where applicable] condition) in (B), (C), (E), (H), and (J)–(L). See also Figure S6.