Clinical flow cytometry, a hypothesis-driven discipline of modern cytomics.

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Recently, two major books have been published that summarize the historical aspects and recent achievements of practical flow cytometry (1,2). Both emphasize the role played by this newly developed technical discipline in the development of scientific (1) and diagnostic platforms during late 20th-century medicine (2). Indeed, the gray box called a flow cytometer is the result of a multidisciplinary collaboration between engineers, biophysicists, biochemists, histopathologists, molecular cytologists, hematologists, immunologists, and quality controllers, with a more recent contribution from physicians specializing in the human immunodeficiency virus (HIV), oncologists, and epidemiologists (Table 1). The foresight by the “fathers” has been astonishing (1). In his book, Howard Shapiro reminds us that flow cytometry, used to investigate cells in a flow system instead of on a static microscope, was put into practice by Coulter when he used impedance to count red cells. Rapid cell spectroscopy was introduced by Kamentsky to leukocyte differential counting with computer-assisted displays. Then optical cell counters, fluorescence dies, lasers, and photomultiplier tubes were added by Fulwyler and Jones. Immunologic concepts and reagents were introduced by the Herzenbergs and were fully primed to harvest the gems of the incipient monoclonal antibody revolution. The first commercial cytometer with precision engineering of the flow cells was launched by Goehde. These initial steps were duly followed by the release of a series of cytometers that showed increasing sensitivity, computerization, and practicality in research and routine laboratory work. This is illustrated by science historians Keating and Cambrosio in their book that blends the sociological aspects of medical technological developments with witness accounts (2). Flow cytometry currently is a colorful, practical discipline that has become available to medicine at the appropriate time for various pressing clinical applications, thus rewarding the scientists who had the vision to foresee these needs. These contributions established cytometry, including flow technology, image analysis, and advanced microscopy, among the leading trends of modern biomedicine and health care. After the launch of the Human Genome Project (3), two areas, genomics and proteomics, have been majestically promoted on both sides of the Atlantic and in the East (4). Genomics includes the identification of genes and gene regulatory processes, and proteomics investigates the abundance of proteins simultaneously with the changes associated with alterations of the functional state of the cell. Such a “pseudo-functional” approach aims to extend the study of quantitative changes during differentiation, proliferation, and signaling of different cell types (5). Clearly, there is an enormous, newly generated influx of information here, but it is not certain that a mere analysis of genes and protein structure, even in its extended format that includes the interaction of various biomolecules, will provide all of the necessary information to understand function and regulation at the level of living cells and organisms. Hence, the concept of cytomics has been introduced recently (6,7), for two reasons. First, cytomics is the cell-oriented analysis of molecules and their functions in cellular systems and/or organs, referred to as cytomes. Second, cytomics is the new, absolutely essential, interface between biosciences and clinical medicine. Importantly, advanced cytometry, including the traits defined in Table 1, represent the driving engine of cytomics. A new paradigm is that cytomics is the combined, interrelated force for cytometry, proteomics, and genomics (6,7). There are, however, significant differences between these three modern trends. First, it is in the realm of cytometry, with the major contribution of flow systems, where the appropriate functional analysis of cells, the display and regulation of functional molecules, and the differentiation pathways are successfully carried out. Animal models and human diseases provide targets for these studies. Second, it is the cytometric field where the diagnostic tests, for malignancies (cf. 8,9) and the cellular analysis of infectious diseases (10), currently harvest the richest crop of practical results, whereas in the fields of genomics and proteomics only the seeds are sown for the practically useful multiarray systems. Third, these two fields at their current developmental state, are essentially observational sciences that primarily document the observed heterogeneity. As a marked contrast, the research by flow cytometry has been, from the beginning, primarily