Exploring the maturation of a monocytic cell line using self-organizing maps of single-cell Raman spectra

Phorbol myristate acetate (PMA)-differentiated THP-1 cells are routinely used in lieu of primary macrophages to study macrophage polarization during host-pathogen interactions and disease progression. The phenotypes of the THP-1 macrophages are influenced by the level and duration of PMA stimulation, and possibly also by the presence of adhesion factors. Here, we use self-organizing maps (SOMs) of single-cell Raman spectra to probe the effects of PMA stimulation conditions and adhesion factors on THP-1 cell differentiation. Raman spectra encoding for biochemical composition were acquired from individual cells on substrates coated with fibronectin or poly-L-lysine before and after stimulation with 20 nM or 200 nM PMA for two different time intervals. SOMs that show the extent of spectral dissimilarity were constructed. For all stimulation conditions, the SOMs indicated the spectra acquired from cells after 3 d treatment had diverged from those of untreated cells. The SOMs also showed treatment with 200 nM PMA for 3 d produced both fully and partially differentiated cells on both adhesion factors, whereas the outcome of 20 nM PMA treatment for 3 d depended on the adhesion factor. Treatment of THP-1 cells on fibronectin with 20 nM PMA for 3 d produced both partially and fully differentiated cells, but this treatment induced an intermediate stage of differentiation when applied to THP-1 cells on poly-L-lysine. Thus, the transition of THP-1 monocytes into macrophage-like cells may be modulated by integrin-binding interactions. Furthermore, the composite of culture and stimulation conditions may confound the comparison of results from separate studies.