P. V. Sivakumar

Natural killer cell differentiation: insights from knockout and transgenic mouse models and in vitro systems

Summary: In the last few years, the routine development of knockout and transgenic mice and the ease with which rare progenitor populations can be isolated from hematopoietic organs and cultured in vitro has facilitated significant advances in understanding the lineage and development of natural killer (NK) cells. Fluorescence‐activated cell sorter analyses have identified a common lymphoid progenitor capable of giving rise to NK, T, and B cells, confirming the lymphoid origin of NK cells. Knockout and transgenic mouse models have pointed to an absolutely critical role for signals sent through the interleukin (IL)‐2/lS receptor β (CD 122) chain and common γ (γc) chain for NK development. Such signals are likely relayed inside the cell by the tyrosine kinase Jak3, which associates with γc. Recently developed IL‐15 and IL‐15 receptor a knockout mice have pinpointed IL‐15 as the mediator of this signal. Other mouse models have indicated an unexpected role for flt3 ligand in early NK‐cell development as well as minor roles for stem cell factor and IL‐7 in expanding NK‐cell progenitor numbers. Finally, in vitro culture systems have proven useful in identifying the point in NK development at which each of these signals is critical.

Development of Self-Recognition Systems in Natural Killer Cells

Differentiation of NK cells is bone marrow dependent. Although all the factors necessary for NK differentiation are yet to be fully characterized, IL-15 has emerged as the most likely candidate that drives terminal differentiation of NK cells. Other cytokines are needed for the expansion and maintenance of the progenitor population. Although the in vivo role for IL-15 cannot be established without the generation of either IL-15 or IL-15R alpha deficient mice, in vitro data suggests that it is responsible for the generation of lytic, NK1.1+ cells from immature progenitors. So far, it has not been possible to obtain Ly-49+ cells from marrow or fetal-derived progenitor cells in vitro. Stromal cells along with cytokines may be necessary to induce expression of Ly-49 on NK1.1+ cells. Expression of the NK receptors seems to be a sequential process with expression of IL-2/15R beta on progenitor cells occurring first followed by the expression of NK1.1 and then probably Ly-49. The same sequence seems to hold true in vivo as well, Ly-49 surface expression on splenic NK1.1+ cells is first detected 4-6 days after birth, and the frequency of cells expressing Ly-49 receptors reaches adult levels by days 20-24. Despite the lack of expression of Ly-49 receptors by fetal NK1.1+ as well as bone marrow derived NK1.1+ cells, they are able to distinguish between MHC class Ihi and class Ilo targets. This suggests that these NK1.1+Ly-49- cells express non-Ly-49 class I receptors. Efforts in the future need to be focused on elements responsible for the expression of Ly49 on these NK1.1+ cells in order to establish an in vitro system in which establishment of the Ly-49 repertoire can be studied.

Ontogeny and Differentiation of Murine Natural Killer Cells and Their Receptors

Cellular immunology was actively investigated during the 1960s, stimulated in large part by the development of plaque assays for antibody-forming cells (Jerne and Nordin 1963) and the assay for cytotoxic cells (Rosenau and Moon 1964). The discovery that the thymus and bursa of Fabricius are necessary for the development of T and B cells (Miller and Mitchell 1969; Glick and Whatley 1967; Cooper et al. 1965) led to the concept of “central lymphoid organs.” Subsequently it appeared that the effector cell responsible for rejection of bone marrow allografts following large doses of total-body irradiation is not a T or a B cell, but is eliminated by destruction of bone marrow with the bone-seeking isotope 89Sr (Cudkowlcz and Bennett 1971; Bennett 1973). Thus these effector cells were termed “marrow-dependent” or M cells, based on the analogy with the thymus and bursa-dependent T and B cell. Later studies revealed that M cells are identical to natural killer (NK) cells.