528 PART IV The Immune System in Health and Disease then be grown in a chemically defined basal medium(on- SCID mouse taining saline, sugars, amino acids, vitamins, trace elements, and other nutrients) to which various serum supplements are added. For some experiments, serum-free culture condi tions are employed. Because in vitro culture techniques re o Transplant human thymus quire from 10-to 100-fold fewer lymphocytes than do typical and lymph-node tissue in vivo techniques, they have enabled immunologists to under kidney capsule the functional properties of I lymphocytes. It was by means of cell-culture techniques, liver cells(stem cells) example, that immunologists were first able to define the functional differences between CD4* T helper cells and CD8 T cytotoxic cells. Cell-culture techniques have also been used to identify numerous cytokines involved in the activation, growth, and differentiation of various cells involved in the immune re- nents showed that media conditioned, 米a④ Human thymus releases or modified, by the growth of various lymphocytes or antigen- presenting cells would support the growth of other lymphoid of--A cells Conditioned media contain the secreted products from actively growing cells. Many of the individual cytokines that characterized various conditioned media have subsequently been identified and purified, and in many cases the genes that encode them have been cloned. These cytokines, which play a SCID-human mouse central role in the activation and regulation of the immune response, are described in Chapter 12 and elsewhere FIGURE 23-1 Production of SCID-human mouse. This system Cloned Lymphoid Cell Lines permits study of human lymphocytes within an animal model. In this example, human T-cells are transferred to SCID mouse, but B-cells A primary lymphoid cell culture comprises a heterogeneous also can be transferred by the use of bone-marrow precursors group of cells that can be propagated only for a limited time This heterogeneity can complicate the interpretation of perimental results. To avoid these problems, immunologists use cloned lymphoid cell lines and hybrid cells. Normal mammalian cells generally have a finite life span in The beauty of the SCID-human mouse is that it enables culture; that is, after a number of population doublings char one to study human lymphocytes within an animal model. acteristic of the species and cell type, the cells stop dividing. In This valuable system has proved useful in research on the contrast, tumor cells or normal cells that have undergone development of various lymphoid cells and also as an impor- transformation induced by chemical carcinogens or viruses cytes cannot be infected with HIV, whereas the lymphocytes said to be immortal. Such cells are referred to as cell ir / e tant animal model in AIDS research, since mouse lympho- can be propagated indefinitely in tissue culture; thus, they are of a SCiD-human mouse are readily infected. The first cell line-the mouse fibroblast l cell-was de. ived in the 1940s from cultured mouse subcutaneous con- nective tissue by exposing the cultured cells to a chemical carc Cell-Culture Systems nogen, methylcholanthrene, over a 4-month period. In the 1950s, another important cell line, the Hela cell was de The complexity of the cellular interactions that generate an rived by culturing human cervical cancer cells. Since these immune response has led immunologists to rely heavily on early studies, hundreds of cell lines have been established,each various types of in vitro cell-culture systems. A variety of cells consisting of a population of genetically identical (syngeneic) can be cultured, including primary lymphoid cells, cloned cells that can be grown indefinitely in culture lymphoid cell lines, and hybrid cells. Table 23-3 lists some of the cell lines used in immunologic research and briefly describes their properties. Some were Primary Lymphoid Cell Cultures derived from spontaneously occurring tumors of lympho- cytes, macrophages, or other cells involved in the im Primary lymphoid cell cultures can be obtained by isolating mune response. In other cases, the cell line was induced by lymphocytes directly from blood or lymph or from various transformation of normal lymphoid cells with viruses such as lymphoid organs by tissue dispersion. The lymphocytes can Abelson's murine leukemia virus(A-MLV), simian virus 40The beauty of the SCID-human mouse is that it enables one to study human lymphocytes within an animal model. This valuable system has proved useful in research on the development of various lymphoid cells and also as an impor￾tant animal model in AIDS research, since mouse lympho￾cytes cannot be infected with HIV, whereas the lymphocytes of a SCID-human mouse are readily infected. Cell-Culture Systems The complexity of the cellular interactions that generate an immune response has led immunologists to rely heavily on various types of in vitro cell-culture systems. A variety of cells can be cultured, including primary lymphoid cells, cloned lymphoid cell lines, and hybrid cells. Primary Lymphoid Cell Cultures Primary lymphoid cell cultures can be obtained by isolating lymphocytes directly from blood or lymph or from various lymphoid organs by tissue dispersion. The lymphocytes can then be grown in a chemically defined basal medium (con￾taining saline, sugars, amino acids, vitamins, trace elements, and other nutrients) to which various serum supplements are added. For some experiments, serum-free culture condi￾tions are employed. Because in vitro culture techniques re￾quire from 10- to 100-fold fewer lymphocytes than do typical in vivo techniques, they have enabled immunologists to assess the functional properties of minor subpopulations of lymphocytes. It was by means of cell-culture techniques, for example, that immunologists were first able to define the functional differences between CD4+ T helper cells and CD8+ T cytotoxic cells. Cell-culture techniques have also been used to identify numerous cytokines involved in the activation, growth, and differentiation of various cells involved in the immune re￾sponse. Early experiments showed that media conditioned, or modified, by the growth of various lymphocytes or antigen￾presenting cells would support the growth of other lymphoid cells. Conditioned media contain the secreted products from actively growing cells. Many of the individual cytokines that characterized various conditioned media have subsequently been identified and purified, and in many cases the genes that encode them have been cloned. These cytokines, which play a central role in the activation and regulation of the immune response, are described in Chapter 12 and elsewhere. Cloned Lymphoid Cell Lines A primary lymphoid cell culture comprises a heterogeneous group of cells that can be propagated only for a limited time. This heterogeneity can complicate the interpretation of ex￾perimental results. To avoid these problems, immunologists use cloned lymphoid cell lines and hybrid cells. Normal mammalian cells generally have a finite life span in culture; that is, after a number of population doublings char￾acteristic of the species and cell type, the cells stop dividing. In contrast, tumor cells or normal cells that have undergone transformation induced by chemical carcinogens or viruses can be propagated indefinitely in tissue culture; thus, they are said to be immortal. Such cells are referred to as cell lines. The first cell line—the mouse fibroblast L cell—was de￾rived in the 1940s from cultured mouse subcutaneous con￾nective tissue by exposing the cultured cells to a chemical carcinogen, methylcholanthrene, over a 4-month period. In the 1950s, another important cell line, the HeLa cell, was de￾rived by culturing human cervical cancer cells. Since these early studies, hundreds of cell lines have been established, each consisting of a population of genetically identical (syngeneic) cells that can be grown indefinitely in culture. Table 23-3 lists some of the cell lines used in immunologic research and briefly describes their properties. Some were derived from spontaneously occurring tumors of lympho￾cytes, macrophages, or other accessory cells involved in the im￾mune response. In other cases, the cell line was induced by transformation of normal lymphoid cells with viruses such as Abelson’s murine leukemia virus (A-MLV), simian virus 40 528 PART IV The Immune System in Health and Disease SCID mouse Transplant human thymus and lymph-node tissue under kidney capsule Inject with human fetal liver cells (stem cells) Stem cells migrate to the human thymus Human thymus releases mature human T cells into circulation SCID–human mouse FIGURE 23-1 Production of SCID-human mouse. This system permits study of human lymphocytes within an animal model. In this example, human T-cells are transferred to SCID mouse, but B-cells also can be transferred by the use of bone-marrow precursors