Accurate detection of low levels of fluorescence emission in autofluorescent background: francisella-infected macrophage cells.

Cellular autofluorescence, though ubiquitous when imaging cells and tissues, is often assumed to be small in comparison to the signal of interest. Uniform estimates of autofluorescence intensity obtained from separate control specimens are commonly employed to correct for autofluorescence. While these may be sufficient for high signal-to-background applications, improvements in detector and probe technologies and introduction of spectral imaging microscopes have increased the sensitivity of fluorescence imaging methods, exposing the possibility of effectively probing the low signal-to-background regime. With spectral imaging, reliable monitoring of signals near or even below the noise levels of the microscope is possible if compensation for autofluorescence and background signals can be performed accurately. We demonstrate the importance of accurate autofluorescence modeling and the utility of spectral imaging and multivariate analysis methods using a case study focusing on fluorescence confocal spectral imaging of host-pathogen interactions. In this application fluorescent proteins are produced when Francisella novicida invade host macrophage cells. The resulting analyte signal is spectrally overlapped and typically weaker than the cellular autofluorescence. In addition to discussing the advantages of spectral imaging for following pathogen invasion, we present the spectral properties and cellular origin of macrophage autofluorescence.

Microfluidic-based cell sorting of Francisella tularensis infected macrophages using optical forces.

We have extended the principle of optical tweezers as a noninvasive technique to actively sort hydrodynamically focused cells based on their fluorescence signal in a microfluidic device. This micro fluorescence-activated cell sorter (microFACS) uses an infrared laser to laterally deflect cells into a collection channel. Green-labeled macrophages were sorted from a 40/60 ratio mixture at a throughput of 22 cells/s over 30 min achieving a 93% sorting purity and a 60% recovery yield. To rule out potential photoinduced cell damage during optical deflection, we investigated the response of mouse macrophage to brief exposures (<4 ms) of focused 1064-nm laser light (9.6 W at the sample). We found no significant difference in viability, cell proliferation, activation state, and functionality between infrared-exposed and unexposed cells. Activation state was measured by the phosphorylation of ERK and nuclear translocation of NF-kappaB, while functionality was assessed in a similar manner, but after a lipopolysaccharide challenge. To demonstrate the selective nature of optical sorting, we isolated a subpopulation of macrophages highly infected with the fluorescently labeled pathogen Francisella tularensis subsp. novicida. A total of 10,738 infected cells were sorted at a throughput of 11 cells/s with 93% purity and 39% recovery.