Cryptomonad algal phycobiliproteins as fluorochromes for extracellular and intracellular antigen detection by flow cytometry

Background Phycobiliproteins play an important role in fluorescent labeling, particularly for flow cytometry. The spectral properties of R‐phycoerythrin (R‐PE) and allophycocyanin (APC) have made them the dominant reagents in this class of fluorochromes. In this study, we evaluate a lesser‐known but potentially important series of low‐molecular weight cryptomonad‐derived phycobiliproteins (commercially termed the CryptoFluor™ dyes) for their applicability to flow cytometry, both in extracellular and intracellular labeling applications. Methods Several cell lines were labeled with biotin‐conjugated antibodies against expressed extracellular surface proteins, followed by streptavidin conjugates of three cryptomonad phycobiliproteins (CryptoFluor‐2, CryptoFluor‐4, and CryptoFluor‐5). Cells were then analyzed by flow cytometry using a variety of laser lines and emission filters to establish the optimal excitation/emission characteristics for each fluorochrome. Some cells were permeabilized and labeled for intracellular antigens, also using the cryptomonad fluorochromes. Where appropriate, parallel samples were labeled with other fluorochromes (including R‐PE, APC, the cyanin dyes Cy3 and Cy5, and others) to gauge the performance of the cryptomonad fluorochromes against fluorescent labels previously evaluated for flow cytometry. Results CryptoFluor‐2 possessed excitation/emission maxima similar to those of APC and Cy5, with good excitation in the red (HeNe laser 632 nm) and strong emission in the far red (660 nm). CryptoFluor‐4 possessed excitation/emission maxima similar to those of Cy3, with optimal excitation in the green (Kr 530 nm) and strong emission in the yellow/orange (585 nm). CryptoFluor‐5 possessed excitation/emission maxima similar to those of lissamine rhodamine, with optimal excitation in the yellow (Kr 568 nm) and emission in the orange (610 nm). All cryptomonad fluorochromes gave satisfactory results for both intracellular and extracellular labeling, with detection sensitivities that were comparable or better than traditional phycobiliproteins and low‐ molecular weight synthetic fluorochromes such as the cyanin dyes. Conclusions The CryptoFluor fluorochromes were applicable to flow cytometric immunodetection, with excitation and emission conditions commonly found on multilaser instruments. Performance of several of these dyes was at least comparable to existing fluorescent labels. The low molecular weights (30–60 kd) of phycobiliproteins may ma