Immature mouse granulocytic myeloid cells are characterized by production of ficolin-B

Ficolins activate the lectin pathway of the complement system upon binding to carbohydrate patterns on pathogens. To characterize the producer cells of ficolin-B the expression of mouse ficolin-B, the orthologue of human M-ficolin, was studied in macrophages and dendritic cells during differentiation from bone marrow cells, in primary granulocytes, and during differentiation of granulocytes derived from ER-Hoxb8 cells. Expression of ficolin-B mRNA declined in all myeloid cell types to low levels during terminal differentiation. However, in contrast to macrophages and dendritic cells, ficolin-B expression was enhanced upon activation in granulocytes. High expression of ficolin-B was observed in primary immature neutrophilic CD11b(+) Ly-6C(int) Ly-6G(high) granulocytes when isolated from the bone marrow, in particular during sepsis. Ficolin-B was demonstrated in lysates of primary granulocytes, ER-Hoxb8-derived granulocytes, bone marrow-derived macrophages, and dendritic cells. Native ficolin-B from cell lysates and supernatants of granulocytes activated the lectin pathway as measured by binding to MASP-2 and inducing C4 deposition. Specific staining demonstrated intra-cellular or cell associated ficolin-B protein in activated immature granulocytes deposited in a granular fashion. This study shows that ficolin-B is stored in and set free from immature granulocytic myeloid cells indicating a role in the early infection-induced cellular response of these inflammatory cells.

Mycobacterium tuberculosis sigma factor E regulon modulates the host inflammatory response

Mycobacterium tuberculosis survives in macrophages and usually subverts the bactericidal mechanisms of these phagocytes. The understanding of this host-pathogen interaction is relevant for the development of new treatments for tuberculosis. The adaptation of M. tuberculosis to intracellular life depends on its ability to regulate the expression of its genes. Sigma factors are important bacterial transcription activators that bind to the RNA polymerase and give it promoter specificity. Sigma factor E (SigE) controls the expression of genes that are essential for virulence. We have identified the SigE regulon during infection of macrophages, and we analyzed the impact of this regulon on the transcriptional response of phagocytes. Our results indicate that SigE regulates the expression of genes involved in the maintenance of M. tuberculosis cell envelope integrity and function during macrophage infection. Analysis of the phagocytes' transcriptional response indicates that the SigE regulon is involved in the modulation of the inflammatory response.

Downregulation of mac-1 expression in monocytes by surface-bound IgG

Physical and functional association between the beta2-integrin Mac-1 (CD11b/CD18) and receptors of immunoglobulin G (IgG) (FcgammaRs) has been previously reported. In this study, we examined the modulation of Mac-1 expression by IgG in different leucocyte populations. Our data show that human monocytes, but not neutrophils, macrophages, dendritic or natural killer cells, downregulate the expression of Mac-1 after overnight exposure to surface-bound IgG. This effect, which requires at least 6 h of incubation, is not associated with a general downmodulation of membrane antigens, and is selectively induced by immobilized IgG (iIgG), as the stimulation of monocytes with N-formyl-methionyl-leucyl-phenylalanine, lipopolysaccharide, tumour necrosis factor-alpha (TNF-alpha) or soluble IgG did not modify the Mac-1 expression after 18 h in culture. The loss of Mac-1 was completely prevented by blocking antibodies (Abs) directed to FcgammaRII (CD32) or CD18. On the other hand, the serine protease inhibitor, phenyl methyl sulphonyl fluoride, but not inhibitors of cysteine proteases or neutral endopeptidases, partially prevented the downregulation of Mac-1 by iIgG. Monocytes cultured overnight on iIgG exhibited a dramatic decrease in their capacity to ingest zymosan particles that could be attributed to the reduced expression of Mac-1. However, there was no inhibition of TNF-alpha production induced by zymosan, suggesting that Mac-1-dependent responses require different levels of its expression to be fully activated.

Non-malignant leukocytes delay spontaneous B-CLL cell apoptosis

Malignant B cells from chronic lymphocytic leukemia (B-CLL) patients have a long survival in vivo, although, in culture, they spontaneously die by apoptosis. Here, we analyzed the capacity of accessory leukocytes to modulate apoptosis of B-CLL cells in vitro. To this end, we performed long-term cultures using total mononuclear cells (TMC) from B-CLL patients and TMC depleted from monocytes, NK cells and T lymphocytes (B-CLL cells). In all the patients studied (n = 25) the presence of accessory leukocytes markedly prolonged the survival of B-CLL cells. The anti-apoptotic effect was exerted by monocytes and, to a lesser degree, NK cells, partially through the release of soluble factors. Indeed, accessory leukocytes separated from leukemic cells by semipermeable membranes were still able to prolong B-CLL cell survival. By flow cytometric analysis we found that the protective effect of non-malignant cells was associated with delayed down-regulation of Bcl-2 expression on leukemic cells. By contrast, the expression of Fas and Fas ligand proteins was unchanged in most samples. Our findings suggest that monocytes and NK cells, by delaying leukemic cell apoptosis, may play a role in B-CLL cell accumulation in vivo.

Immune complexes inhibit apoptosis of chronic lymphocytic leukaemia B cells

In the present study we examined the effect of immune complexes (IC) on the survival of chronic lymphocytic leukaemia (B-CLL) B cells. Our results showed that either precipitating IC (pIC), Ab-coated erythrocytes (E-IgG) or heat-aggregated IgG (aIgG) significantly inhibited spontaneous apoptosis of B-CLL cells, as well as that induced by fludarabine, chlorambucil or dexamethasone. After depletion of T lymphocytes, monocytes and NK cells, incubation with IC was no longer able to delay B-CLL cells apoptosis, suggesting that prevention of apoptosis depends on IC interaction with accessory leucocytes. The release of IFNgamma by non-malignant cells upon activation with IC was responsible, to some extent, for IC effects as shown by the fact that neutralizing anti-IFNgamma MoAb partially prevented their ability to inhibit B-CLL cells apoptosis. The observation that treatment with IC resulted in increased expression of HLA-DR on B-CLL cells suggests that inhibition of apoptosis is associated with cellular activation.