George Hodgson

Multiple defects in the immune system of Lyn-deficient mice, culminating in autoimmune disease

Mice homozygous for a disruption at the Lyn locus display abnormalities associated with the B lymphocyte lineage and in mast cell function. Despite reduced numbers of recirculating B lymphocytes, Lyn-/- mice are immunoglobulin M (IgM) hyperglobulinemic. Immune responses to T-independent and T-dependent antigens are affected. Lyn-/- mice fail to mediate an allergic response to IgE cross-linking, indicating that activation of LYN plays an indispensable role in Fc epsilon RI signaling. Lyn-/- mice have circulating autoreactive antibodies, and many show severe glomerulonephritis caused by the deposition of IgG immune complexes in the kidney, a pathology reminiscent of systemic lupus erythematosus. Collectively, these results implicate LYN as having an indispensable role in immunoglobulin-mediated signaling, particularly in establishing B cell tolerance.

Gain- and Loss-of-Function Lyn Mutant Mice Define a Critical Inhibitory Role for Lyn in the Myeloid Lineage

To investigate the role of the Lyn kinase in establishing signaling thresholds in hematopoietic cells, a gain-of-function mutation analogous to the Src Y527F-activating mutation was introduced into the Lyn gene. Intriguingly, although Lyn is widely expressed within the hematopoietic system, these mice displayed no propensity toward hematological malignancy. By contrast, analysis of aging cohorts of both loss- and gain-of-function Lyn mutant mice revealed that Lyn(-/-) mice develop splenomegaly, increased numbers of myeloid progenitors, and monocyte/macrophage (M phi) tumors. Biochemical analysis of cells from these mutants revealed that Lyn is essential in establishing ITIM-dependent inhibitory signaling and for activation of specific protein tyrosine phosphatases within myeloid cells. Loss of such inhibitory signaling may predispose mice lacking this putative protooncogene to tumorigenesis.

Granulocyte-colony stimulating factor is not critical for mobilisation of granulocytes from bone marrow into circulation

Abstract In normal steady-state hematopoiesis, G-CSF not only regulates the production of neutrophils, it is also involved in release of neutrophil from bone marrow into the circulation. In the present study we have evaluated the physiological role of G-CSF in the release of granulocytes from marrow into circulation. The thymidine analogue, 5′-bromo-2′-deoxyuridine (BrdU), was used to label dividing bone marrow cells and follow the release of granulocytes into circulation in G-CSF-deficient and wild type mice. The bone marrow and blood cells were labeled with anti-BrdU antibody conjugated to FITC and anti-Gr-1 antibody conjugated to PE (marker for granulocytes) and analysed by FACScan. Interestingly, the labeling index and the amount of BrdU incorporated by bone marrow cells was greater in G-CSF-deficient mice compared to wild type mice. This phenomenon was most striking in the blast cell population clearly indicating that this population of cells is cycling faster in G-CSF-deficient mice compared to wild type mice. However, a greater proportion of early granulocytic cells in the bone marrow was undergoing apoptosis in G-CSF-deficient mice compared to wild type mice. The kinetics of release of granulocytes from marrow into circulation was similar in both G-CSF and wild type mice. Similarly, the transit time of granulocytes through the mitotic and post-mitotic pool in the marrow was unaffected in absence of G-CSF. Furthermore, the peripheral half-lives of granulocytes of G-CSF-deficient and wild-type mice were comparable. These findings demonstrate that while G-CSF is important for survival of granulocytic cells, under physiological conditions, G-CSF is dispensable for mobilisation of granulocytes from marrow into circulation.