Population dynamics of "null" and Thy1lo lymphocytes in mouse bone marrow: genesis of cells with natural killer cell lineage characteristics

Posttranscriptional regulation of II10 gene expression allows natural killer cells to express immunoregulatory function

Natural killer (NK) cells play a well-recognized role in early pathogen containment and in shaping acquired cell-mediated immunity. However, indirect evidence in humans and experimental models has suggested that NK cells also play negative regulatory roles during chronic disease. To formally test this hypothesis, we employed a well-defined experimental model of visceral leishmaniasis. Our data demonstrated that NKp46(+)CD49b(+)CD3(-) NK cells were recruited to the spleen and into hepatic granulomas, where they inhibited host protective immunity in an interleukin-10 (IL-10)-dependent manner. Although IL-10 mRNA could be detected in activated NK cells 24 hr after infection, the inhibitory function of NK cells was only acquired later during infection, coincident with increased IL-10 mRNA stability and an enhanced capacity to secrete IL-10 protein. Our data support a growing body of literature that implicates NK cells as negative regulators of cell-mediated immunity and suggest that NK cells, like CD4(+) T helper 1 cells, may acquire immunoregulatory functions as a consequence of extensive activation.

Turnover and proliferation of NK cells in steady state and lymphopenic conditions

To gain insight into NK cell dynamics, we investigated the turnover and proliferation rates of NK cells in normal and lymphopenic conditions. In contrast to previous reports suggesting a very rapid turnover of NK cells, continuous 5-bromo-2'-deoxyuridine (BrdU)-labeling studies demonstrated that the time necessary for labeling 50% of splenic NK cells in mature mice was 17 days, similar to the rate of labeling of memory T cells. In contrast, in young mice, splenic NK cells labeled very rapidly with BrdU, although cell cycle analyses and BrdU pulse-labeling studies suggested that most of this proliferation occurred in a precursor population. A somewhat larger percentage of bone marrow NK cells was cycling, suggesting that these proliferating cells are the precursors of the mostly nondividing or slowly dividing splenic NK cells. Splenic NK cells from mature mice also did not proliferate significantly when transferred to normal mice, but did proliferate when transferred to irradiated mice. Thus, NK cells, like T cells, undergo homeostatic proliferation in a lymphopenic environment. Homeostatic proliferation of NK cells was not dependent on host cell class I molecules or host production of IL-15. Nevertheless, the number of recovered NK cells was much lower in IL-15(-/-) hosts. These results suggest that IL-15 is not essential for homeostatic proliferation of NK cells, but is necessary for survival of the NK cells. Our results provide important basic information concerning the production and replacement of NK cells.

The effect of immunization with killed tumor cells, with/without feeding of Echinacea purpurea in an erythroleukemic mouse model

OBJECTIVE

Tumor amelioration via vaccination/immunization is a practice for which considerable experimental and clinical support is growing. Combination therapies have proven to be more beneficial than treatment with single agents. We hypothesized that immunization of mice with killed erythroleukemia cells prior to the induction of erythroleukemia via injection of viable tumor cells, plus dietary administration of a known immuno-enhancing phytocompound, Echinacea purpurea, would be more effective than immunization alone.

DESIGN

A commercially available extract of E. purpurea root, already proven as a natural killer (NK) cell stimulant, was administered via the chow, for periods of 9 days or 3 months after the onset of leukemia to mice which had been injected (immunized) 5 weeks earlier with killed leukemia cells.

RESULTS

Immunized mice (+/- E. purpurea) had significantly prolonged life spans versus non-immunized mice, with an even greater proportion of hosts surviving long-term in the E. purpurea-fed group. NK cells, the mediators of nonspecific immunity and well-demonstrated mediators of tumor cytolysis, were very significantly elevated in immunized, leukemic mice receiving E. purpurea in their diet versus those receiving untreated chow. Early in tumor development (9 days), cells mediating specific immunity (T, B lymphocytes) were 10-12 times higher in absolute numbers in the spleens in all immunized, leukemic mice vs unimmunized, leukemic mice at the same stage of tumor progression.

CONCLUSIONS

The results demonstrate that combination therapy, involving specific tumor cell immunization, followed by daily phytotherapy (dietary E. purpurea), sensitized the immune cells and led to life span prolongation greater than that provided by immunization alone.

Similar rates of production of T and B lymphocytes in the bone marrow

The rate of renewal of T lymphocytes in the bone marrow of euthymic C57BL/Ka and athymic nu/nu BALB/c mice was estimated by in vivo labeling with bromodeoxyuridine. T lymphocytes accounted for 16-18% of marrow cells in euthymic mice as judged by immunofluorescent staining with monoclonal antibodies for Thy-1, CD3, and alpha/beta T cell antigen receptor markers. About 70% of marrow cells expressed receptors (Mac-1, Gr-1, B220) for myeloid, macrophage, and B lineage cells. Approximately 13% of cells in the athymic bone marrow expressed alpha/beta T cell receptors. Sorted marrow T cells proliferated in response to stimulation with anti-alpha/beta antibodies in vitro and showed functional rearrangements of V beta and J beta genes. Sorted non-T cells did not respond to stimulation in vitro, and all V beta and J beta gene rearrangements identified were nonfunctional. In vivo labeling studies indicated that approximately 17 x 10(6) bone marrow T cells are renewed daily in euthymic mice and approximately 14 x 10(6) are renewed in athymic mice. Approximately 11 x 10(6) mature B cells (immunoglobulin M+) are renewed daily in the bone marrow of the latter mice. To determine whether marrow precursors can give rise to T cells directly, marrow cells from euthymic and athymic mice were depleted of T cells by cell sorting and incubated in vitro for 48 h in the absence of exogenous growth factors or thymic stromal cells. Examination of the cells after culture showed that 10-12% stained brightly for alpha/beta T cell receptors. Although functional rearrangements of V beta and J beta genes were not detected before culture, the majority of rearrangements were functional after culture. The emergence of the bright alpha/beta T cells in culture was dependent on depletion T cells from the marrow cells before culture. The results suggest that most marrow T cells are generated in the marrow itself.

Changes in the populations of null, NK1.1+, and Thy1lo lymphocytes in the bone marrow of tumor-bearing mice: Effect of indomethacin treatment

Mouse bone marrow produces many "null" lymphocytes which lack B and T lineage markers (B220-Thy1-). A subset of these cells expresses the natural killer (NK) cell marker, NK1.1. In addition, some rapidly renewed bone marrow lymphocytes express low intensities of Thy1 (Thy1lo). In view of their possible implication in tumor-host interactions these various cell populations have now been examined in mice injected with either the nonmetastatic Ehrlich ascites (EA) tumor or the Lewis lung carcinoma (LLc), a highly metastatic solid tumor. In each case, the number of null lymphocytes, as defined by a lack of radioautographic labeling of either B220 glycoprotein or Thy1, increased markedly in both the bone marrow and spleen. Treatment with the prostaglandin inhibitor, indomethacin, enhanced the increase in null cells in the bone marrow and spleen of LLc-bearing mice. The number of null small lymphocytes expressing NK1.1, as detected by combined radioautographic and immunoperoxidase techniques, increased almost 30-fold in LLc-bearing mice. The number of Thy1lo small lymphocytes increased in parallel with null cells during EA tumor growth. The findings accord with the hypothesis that the null lymphocyte population produced in mouse bone marrow includes newly formed NK lineage cells which sequentially express NK1.1 and Thy1lo. The present work demonstrates that the populations of null, NK1.1+, and Thy1lo lymphocytes in mouse bone marrow expand rapidly during the early growth of transplanted tumors, the initial increase in null lymphocytes apparently being curtailed by prostaglandin production. The results suggest that the production of null lymphocytes in mouse bone marrow is responsive to tumor development, possibly providing cells to be involved in tumor-host interactions.