Immortalized cells are often used to model the behavior of osteogenic cells on orthopaedic and dental biomaterials. In the current study we compared the adhesive behavior of two osteosarcoma cell lines, MG-63 and Saos-2, with that of mesenchymal stem cells (MSCs) on hydroxyapatite (HA). It was found that osteosarcoma cells demonstrated maximal binding to fibronectin-coated HA, while MSCs alternately preferred HA coated with collagen-I. Interesting, the binding of MG-63 and Saos-2 cells to fibronectin was mediated by both α5 and αv-containing integrin heterodimers, whereas only αv integrins were used by MSCs. Cell spreading was also markedly different for the three cell types. Osteosarcoma cells exhibited optimal spreading on fibronectin, but poor spreading on HA disks coated with fetal bovine serum. In contrast, MSCs spread very well on serum-coated surfaces, but less extensively on fibronectin. Finally, we evaluated integrin expression and found that MSCs have higher levels of α2 integrin subunits relative to MG-63 or Saos-2 cells, which may explain the enhanced adhesion of MSCs on collagen-coated HA. Collectively our results suggest that osteosarcoma cells utilize different mechanisms than MSCs during initial attachment to protein-coated HA, thereby calling into question the suitability of these cell lines as in vitro models for cell/biomaterial interactions.

Pulmonary infections with nontuberculous mycobacteria (P-NTM), such as by Mycobacterium avium complex (M. avium), are increasingly found in the elderly, but the underlying mechanisms are unclear. Recent studies suggest that adaptive immunity is necessary, but not sufficient, for host defense against mycobacteria. Heme oxygenase-1 (HO-1) has been recognized as a critical modulator of granuloma formation and programmed cell death in mycobacterial infections. Old mice (18-21 mo) infected with M. avium had attenuated HO-1 response with diffuse inflammation, high burden of mycobacteria, poor granuloma formation, and decreased survival (45%), while young mice (4-6 mo) showed tight, well-defined granuloma, increased HO-1 expression, and increased survival (95%). To further test the role of HO-1 in increased susceptibility to P-NTM infections in the elderly, we used old and young HO-1 and HO-1 mice. The transcriptional modulation of the JAK/STAT signaling pathway in HO-1 mice due to M. avium infection demonstrated similarities to infected wild-type old mice with upregulation of SOCS3 and inhibition of Bcl2. Higher expression of SOCS3 with downregulation of Bcl2 resulted in higher macrophage death via cellular necrosis. Finally, peripheral blood monocytes (PBMCs) from elderly patients with P-NTM also demonstrated attenuated HO-1 responses after M. avium stimulation and increased cell death due to cellular necrosis (9.69% ± 2.02) compared with apoptosis (4.75% ± 0.98). The augmented risk for P-NTM in the elderly is due, in part, to attenuated HO-1 responses, subsequent upregulation of SOCS3, and inhibition of Bcl2, leading to programmed cell death of macrophages, and sustained infection.

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