The effect of unilateral limb shielding on the haemopoietic response of the guinea-pig to gamma irradiation (150 r.)

A review of radiation countermeasures focusing on injury-specific medicinals and regulatory approval status: part I. Radiation sub-syndromes, animal models and FDA-approved countermeasures

PURPOSE

The increasing global risk of nuclear and radiological accidents or attacks has driven renewed research interest in developing medical countermeasures to potentially injurious exposures to acute irradiation. Clinical symptoms and signs of a developing acute radiation injury, i.e. the acute radiation syndrome, are grouped into three sub-syndromes named after the dominant organ system affected, namely the hematopoietic, gastrointestinal, and neurovascular systems. The availability of safe and effective countermeasures against the above threats currently represents a significant unmet medical need. This is the first article within a three-part series covering the nature of the radiation sub-syndromes, various animal models for radiation countermeasure development, and the agents currently approved by the United States Food and Drug Administration for countering the medical consequences of several of these prominent radiation exposure-associated syndromes.

CONCLUSIONS

From the U.S. and global perspectives, biomedical research concerning medical countermeasure development is quite robust, largely due to increased government funding following the 9/11 incidence and subsequent rise of terrorist-associated threats. A wide spectrum of radiation countermeasures for specific types of radiation injuries is currently under investigation. However, only a few radiation countermeasures have been fully approved by regulatory agencies for human use during radiological/nuclear contingencies. Additional research effort, with additional funding, clearly will be needed in order to fill this significant, unmet medical health problem.

Investigating the Abscopal Effects of Radioablation on Shielded Bone Marrow in Rodent Models Using Multimodality Imaging

The abscopal effect is the response to radiation at sites that are distant from the irradiated site of an organism, and it is thought to play a role in bone marrow (BM) recovery by initiating responses in the unirradiated bone marrow. Understanding the mechanism of this effect has applications in treating BM failure (BMF) and BM transplantation (BMT), and improving survival of nuclear disaster victims. Here, we investigated the use of multimodality imaging as a translational tool to longitudinally assess bone marrow recovery. We used positron emission tomography/computed tomography (PET/CT), magnetic resonance imaging (MRI) and optical imaging to quantify bone marrow activity, vascular response and marrow repopulation in fully and partially irradiated rodent models. We further measured the effects of radiation on serum cytokine levels, hematopoietic cell counts and histology. PET/CT imaging revealed a radiation-induced increase in proliferation in the shielded bone marrow (SBM) compared to exposed bone marrow (EBM) and sham controls. T₂-weighted MRI showed radiation-induced hemorrhaging in the EBM and unirradiated SBM. In the EBM and SBM groups, we found alterations in serum cytokine and hormone levels and in hematopoietic cell population proportions, and histological evidence of osteoblast activation at the bone marrow interface. Importantly, we generated a BMT mouse model using fluorescent-labeled bone marrow donor cells and performed fluorescent imaging to reveal the migration of bone marrow cells from shielded to radioablated sites. Our study validates the use of multimodality imaging to monitor bone marrow recovery and provides evidence for the abscopal response in promoting bone marrow recovery after irradiation.

Regulation and localization of lymphocyte production in the bone marrow

The homeostatic mechanisms which control B lymphocyte renewal in the bone marrow are unknown. Mouse bone marrow produces many small lymphocytes which develop surface IgM and other B lymphocyte properties. Putative precursors show cytoplasmic mu chains but earlier progenitors have been characterized. Some marrow small lymphocytes are long-lived recirculating B and T cells. [3H]Thymidine and IgM labelling in femoral marrow sections suggest that recirculating lymphocytes migrate mainly through the marrow periphery while indigenous lymphocytes may be formed peripherally and migrate centrally as they mature. Thus, the localization of lymphocytes appears to be non-random. The effects of possible regulatory factors on bone marrow small lymphocytes production have been examined by [3H]thymidine labelling and radioautography. Administration of anti-IgM antibodies in vivo eliminates all B lymphocytes but the marrow lymphocyte production rate remains unchanged. After sublethal X-irradiation the marrow shows an over-shoot B lymphocyte production, while the lymphocyte numbers in shielded marrow remain stable. In neonatally thymectomized or congenitally athymic mice marrow lymphocyte production is unaffected. Studies in germ-free and antigen-stimulated mice reveal a basal level of marrow lymphocyte production, normally stimulated non-specifically by environmental factors. Thus, marrow lymphocyte production appears to be basically independent of feedback control from the peripheral B lymphocyte pool or of specific humoral factors, but fluctuates widely after perturbation or when amplified by exogenous stimuli. These findings suggest the importance of microenvironmental factors, as yet undefined, in the regulation of bone marrow lymphocytes.

Apoptosis and macrophage-mediated cell deletion in the regulation of B lymphopoiesis in mouse bone marrow

Studies of cell population dynamics and microenvironmental organization of B lymphopoiesis in the bone marrow of normal mice and in various genetically modified states have shown that cell loss, involving processes of apoptosis and macrophage-mediated cell deletion, is a prominent feature of the primary genesis of B lymphocytes. Balanced against the influence of proliferative stimulants, the programmed death of precursor B cells provides a quantitative control, determining the magnitude of the final output of functional B lymphocytes to the peripheral immune system. The cell loss mechanisms can be readily set in motion by external or systemic influences, making the B-cell output particularly vulnerable to suppression by ionizing irradiation, stress or other systemic mediators. In addition, however, cell loss exerts an important quality control in the formation of the primary B-cell repertoire. The combination of apoptosis and macrophage-mediated deletion, acting at successive stages of B-cell differentiation, efficiently eliminates many precursors having non-productive Ig gene rearrangements, cell cycle dysregulations, and certain autoreactive Ig specificities. Outstanding areas of further work abound. Important questions concern the nature of mechanisms which underlie the processes of B-cell apoptosis and macrophage deletion in bone marrow, the microenvironmental signals involved in B-cell life or death decisions and genetic factors which may override these B-cell culling mechanisms. The answers will be relevant to problems of autoimmune disease, humoral immunodeficiency and B-cell neoplasia.

Highly restricted expression of a stromal cell determinant in mouse bone marrow in vivo

B lymphocyte precursor cells in mouse bone marrow develop in close association with stromal cells which provide essential growth signals. To identify molecules that may normally play a role in this interaction we have examined the in vivo binding of a new monoclonal antibody (mAb) (KMI6) that recognizes a determinant on a bone marrow stromal cell line (BMS2) in vitro. Flow cytometric and radioautographic evaluations revealed that the antigen recognized by KMI6 is represented on the surface of an extremely small number of cells in bone marrow cell suspensions from adult mice. An apparent molecular mass of 110 kD was obtained by surface labeling of a stromal cell clone and immunoprecipitation. Purified mAb KMI6 labeled with 125I was then given intravenously to young C3H/HeJ mice. Unbound mAb was washed out by cardiac perfusion and femoral bone marrow was examined by light and electron microscope radioautography. KMI6 labeling was heavy on the plasma membrane of many stromal cells, especially those located towards the outer subosteal region. The KMI6-labeled stromal cells were usually associated with cells of lymphoid morphology which they often completely surrounded. The labeling was restricted to areas of stromal cell plasma membranes in contact with lymphoid cells. The lymphoid cells themselves, as well as macrophages and other hemopoietic cells, failed to bind mAb KMI6 significantly. Stromal cells in bone marrow depleted of hemopoietic cells by gamma-irradiation (9,5 Gy) bound mAb KMI6 at reduced intensity. The results demonstrate that the KMI6 determinant, a 110-kD protein, is expressed on bone marrow stromal cells in vivo. Its restriction to areas of interaction with lymphoid cells suggests a role in forming microenvironmental niches of B lymphopoiesis. The surface membrane of individual stromal cells may thus be functionally polarized towards interacting B cell precursors and other hemopoietic cells.

Inhibition of natural killer cell-mediated lysis of tumor cells by normal and regenerating bone marrow

The natural killer (NK) cells which can lyse certain tumor cells during brief incubation in vitro have also been postulated to be the cells responsible for natural resistance to transplanted hemopoietic cells in vivo. To test this hypothesis, we have now measured: 1) the ability of bone marrow cells to compete with tumor cells as targets for spleen NK cells and 2) the effect of a brief incubation with spleen cells on the hemopoietic grafting potential of bone marrow cells. Firstly, when CBA/J mouse spleen cells were incubated with 51Cr-labelled YAC tumor cells together with DBA/2 mouse bone marrow cells, tumor cell lysis was reduced compared with incubation of spleen cells with tumor cells alone. Tumor cell lysis was even less when post-irradiation regenerating bone marrow was used. Secondly, C57B1/6 mouse bone marrow cells incubated with an excess of DBA/2 mouse spleen cells showed a reduced ability to produce hemopoietic spleen colonies in irradiated 129/J mice, whereas incubation with either thymus cells or fewer spleen cells produced no such effect. The results show that, when incubated with spleen cells under the conditions of a standard NK cell assay, regenerating bone marrow cells competitively inhibit the killing of YAC tumor cells and bone marrow progenitor cells are rendered ineffective in their hemopoietic colony-forming potential (CFU-s). These findings suggest that certain hemopoietic progenitor cells and YAC tumor cells can both serve as targets for NK cells, consistent with the view that the spontaneous cytolysis of tumor cells in vitro and natural resistance to bone marrow transplantation in vivo are mediated by cells of a common lineage.

Virgin B cell recruitment and the lifespan of memory clones during antibody responses to 2,4-dinitrophenyl-hemocyanin

The extent to which B cells newly formed in the bone marrow contribute to primary and secondary B cell responses was investigated. This was assessed by constructing chimeras between congenic strains of rats differing in their kappa light chain allotype. Recipient animals received 800 cGy whole body irradiation with hind limb shielding to protect a proportion of their hemopoietic capacity. These rats then received 3 X 10(8) kappa allotype-marked thoracic duct lymphocytes from donors previously immunized twice with either dinitrophenylated spider crab (Maia squinada) hemocyanin (DNP-MSH) or MSH alone. The chimeras were immunized with DNP-MSH and the production of anti-DNP antibody of both donor and host origin was measured. In the period immediately after immunization both newly formed host virgin B cells and donor memory B cells gave rise to substantial proportions of the anti-DNP antibody. After this initial period, antibody production became sustained by activation of memory B cells only. The chimeras were reimmunized with DNP-MSH at 32 days after their first immunization. There was again evidence of a brief period of both virgin and memory B cell activation followed by memory B cell activation only. Donor B cell clones remained dominant in the established response throughout the 5 months each chimera was studied. The data are interpreted as indicating two phases of B cell activation. It is suggested that the first phase where both virgin and memory B cells are activated may be associated with antigen presentation on dendritic or interdigitating cells outside follicles. It is argued that the second phase where only memory B cells are activated is more likely to be associated with antigen on follicular dendritic cells.

Pre-B cells in bone marrow: peanut agglutinin binding and separation of cytoplasmic mu chain-bearing cell populations in normal, post-irradiation and polycythemic mice using fluorescence-activated cell sorting

Mouse bone marrow cells exposed to fluorescein-conjugated peanut agglutinin (PNA) showed subsets of highly labeled cells when analyzed in a fluorescence-activated cell sorter. After separating three cell fractions of large and small PNA-binding cells and PNA-nonbinding cells, respectively, the B lymphocyte precursor (pre-B) cells, having cytoplasmic mu chains (c mu) without surface mu chains (s mu), were recovered solely in the PNA-binding fractions. Only a minority of s mu+ small lymphocytes having the lowest densities of s mu bound PNA. Small and large c mu+ s mu- pre-B cell populations were separated in high degrees of purity in the PNA-binding fractions, especially when obtained from bone marrow undergoing lymphoid regeneration after sublethal X-irradiation and during stimulation of lymphocyte production in post-polycythemic erythroid suppression. Characteristic shifts in the size distribution profile of PNA-binding cells reflected changes in the maturation stage of the pre-B cells. The results demonstrate that surface membrane components with strong PNA-binding capacities characterize c mu+ s mu- pre-B cells in the bone marrow during both normal and perturbed primary B lymphocyte genesis. The PNA-binding sites become undetectable soon after the first expression of s mu. This property permits the isolation from the bone marrow of high concentrations of subsets of large and small c mu+ s mu- cells in a viable state suitable for use in further functional studies.

Regulation of B cell development in mouse bone marrow

Cultured bone marrow cells, after in vitro treatment with hydroxyurea (HU) - a DNA synthesis inhibitor which kills cells in the S phase of the cell cycle - generated 40 to 70% more B cells than untreated control cells. This was shown by fluorescent-activated cell sorter analysis of labelled cells using FITC-F(ab')2 rabbit anti-mouse IgM and functional tests with LPS. The maximum increase was reached after 24 hr of incubation with HU while 6 or 2 hr of exposure had less effect. The effect of HU was dose dependent with a maximum at 4 mM. The same increase of B cells was observed with foetal liver cells but not with spleen or lymph node cells after 24 hr of in vitro HU treatment. Dialysed supernatants from HU treated bone marrow, spleen or foetal liver cells were themselves able to augment the B cell maturation in bone marrow cultures (test cells) as compared with supernatants from untreated cells, showing that soluble factors were involved. Preliminary data showed that inhibitory factors for B cell maturation were produced by normal bone marrow, spleen and thymus cells in vitro and their formation was prevented by HU pretreatment or irradiation (2500 R) whereas stimulatory factors were produced by lymph node cells. Cell separation experiments suggested that T cells and/or adherent cells may be involved in the production of these soluble factors. These data suggest that early B cell development may be under homeostatic control.

The role of bone marrow in different phases of the cellular repopulation of irradiated mouse thymus

In one series of experiments, the cellular repopulation of the thymus was investigated in mice exposed first to 200 R on the whole body and, after various intervals, to 700 R with one leg protected or unprotected during the exposure. When no protection was made, the mice were transplanted with syngeneic bone marrow cells in a defined number immediately after irradiation. Repopulation was fastest when the interval between exposures was 5 days, and most delayed when it was 14 days; with a 30 day interval the speed of repopulation was intermediate, and resembled that of a control group exposed to only the second dose. In another experimental series, thymus repopulation was studied after exposure of mice first to 200 R with one leg protected or unprotected and, after an interval of 5, 14 or 30 days, to a second dose of 700 R on the whole body. In all cases, syngeneic bone marrow cells were grafted intravenously after the second irradiation. The thymus repopulation was enhanced by protection of the leg when 14 day interval separated the exposures. In the other cases, no enhancement was noted. The findings were interpreted to indicate that the cellular composition of the thymus and, in particular, the frequency of the proliferating stem cells at the time of the exposure determines thymic repopulation for about two weeks after irradiation. After this period, repopulation is due to new precursors from the bone marrow which seeded the thymus.