<i>L</i>. <i>major</i> undergoes delayed proliferation until transitioning into monocytes that are permissive for parasite replication.

<p><b>(A and B)</b> LysM-gfp mice were exposed to the bites of <i>L</i>. <i>major-</i>RFP infected sand flies and individual inoculation sites were sequentially imaged employing 2P-IVM. <b>(A)</b> Representative images from the same bite site of a putative single infected cell from 1 to 6 days post-bite. <b>(B)</b> Enumeration of the number of parasites and infected cells (parasites are assumed to be intracellular) per individual bite site employing slice-by-slice visual analysis of the z-stack to determine parasite and infected cell number. Data are the Mean +/- SEM, n = 7 bite sites. <b>(C-E)</b> CX₃CR1-gfp mice were needle inoculated with 2x10⁵ <i>L</i>. <i>major</i>-RFP parasites in the ear. Sixty hours post-infection CD11b⁺Ly6C^int/hiCX₃CR1⁺Ly6G^-RFP⁺ cells were cell-sorted and placed in <i>in-vitro</i> culture. <b>(C)</b> Post-sort analysis of the RFP⁺ sorted population. <b>(D)</b> Quantitative cytospin analysis of the total number of amastigotes per 25 RFP⁺ monocytes (left panel; 100 total monocytes per time point, a denominator of 25 was chosen arbitrarily) or the number of amastigotes per individual monocyte (right panel; n = 100 (d0); 123 (24h) or 179 (48 h), total monocytes). Data are the mean +/- SD. Data are from one experiment representative of 3 similar experiments. <b>(E)</b> Representative cytospin images at 0h and 48h of RFP⁺ murine monocytes. <b>(F-J)</b> <i>In-vitro L</i>. <i>major</i> infection of human elutriated monocytes. <b>(F)</b> Percent infected, number of parasites per each infected, and total number of <i>L</i>. <i>major</i> per 100 total, monocytes. Each data set is an individual experiment employing an individual donor (n = 8), employing ≥108 infected monocytes per time point per donor. <b>(G)</b> Representative cytospin images. <b>(H-J)</b> Respiratory burst in human monocytes employing DHR. <b>(H)</b> Representative histogram of DHR expression by total elutriated monocytes under the indicated conditions. <b>(I)</b> % DHR⁺ monocytes under the indicated conditions. <b>(J)</b> Monocytes were identified as uninfected or infected based upon RFP expression (left panel) and the frequency of DHR⁺ cells (middle panel) was determined (right panel). In (H-J) data are from 5 experiments employing 5 individual donors.