Multicolor Flow Cytometry in Medical Research: A Versatile Tool to Evaluate the Rare Immune Cells in the Pathogenesis of Chronic Viral Infections

Flow cytometry is a versatile tool to study and quantify several properties of cells, including the rarest of cellular populations like circulating dendritic cells (DCs) and/or the recently discovered memory stem T-cells (Tscms) and multipotent progenitor cells, NKT cells, gamma-delta T cells etc. The DCs play an important role as the immune system’s sentinels in initiating immune responses and establishing immunological memory after a pathogen attack, including the newly emerging viruses. Chronic viral infections like HIV-1, Hepatitis C (HCV), Hepatitis B Virus (HBV) and dengue virus are endemic in many countries of the world and remain the most perilous of infectious disease challenges to public health. Recent findings from our and many other labs have shown that chronic viral infections have the capacity to alter critical regulatory mechanisms within dendritic cells (DCs), making them dysfunctional over the course of infection. The flow cytometry was effectively used not only to assess the phenotype and frequency of DCs in the peripheral blood of chronically infected patients but also the functional ability of these cells in circulation. This sophisticated technique has also made possible the study of another rare cell population, the Tscm, which serves as reservoirs of HIV and remain an important hurdle to the complete elimination of the HIV virus from the host. Further, flow cytometry was used to purify these cells to study the intrinsic signaling pathways so they could be targeted further for therapeutic purposes. This chapter will describe the usefulness of flow cytometry-based analysis and sorting, which could be effectively used to decipher some of such mechanisms. The chapter also highlights the important clinical implications related to these mechanisms because the understanding of mechanics in such rare cellular populations is likely to pave the way for designing novel therapeutics in the form of immune-modulators that may lead to better management of these dreaded infections.