Surface Functionalities are Related to Biological Response and Transport of Crystalline Silica

Clearance of silica is a complex process initiated upon deposition in the deep lung. We have examined the relationship of specific surface groups (silanol, siloxane, peroxobridges, surface radicals) to accumulation, clearance and transport of silica.

Among the surface functionalities, some are related to particle–membrane interactions and others to activation of lung cells accompanied by secretion of inflammatory mediators leading to the development of fibrosis.

Alpha-quartz (TAFQ) and α-cristobalite (CRS), were heated in air at 800°C for 24 h. Rats were exposed to aerosols 6 h per day, for 8 days.

Heated CRS elicited an increase in alveolar inflammatory cells. During the exposure period, heated CRS was accumulated at higher levels and remained higher up to 6 months. Alveolar cells from heated CRS exposed lungs contained a higher per cent of the total SiO₂. Heat treatment of TAFQ had no effect on tissue compartments but with heated TAFQ, there was a decreasing amount in the alveolar compartment with time after exposure. Heated CRS was transported to the thymus and mediastinal lymph nodes in a greater amount than was unheated. There was no transport of unheated TAFQ to the lymphoid tissue but there was marked transport with heated TAFQ.

Silanols and surface radicals both decreased upon thermal treatment, leaving isolated silanols, siloxanes and distorted bridges on both particles. The energy interaction with water vapour indicated a transformation of the surface from a hydrophilic to a hydrophobic surface. This means that nearby silanols diappear and randomly, isolated silanols and siloxanes remain at the surface. EPR spectra show that the radical population decreased as a function of increasing temperature with few radicals remaining after treatment. The silanol–siloxane groups appear to be linked to the phagocytosis and transport of particles, whereas the surface radicals and distorted bridges might be more closely associated with the fibrogenic response. Data indicated that heated TAFQ and CRS were not as haemolytic as the unheated materials, in agreement with the hypothesis that it is hydrogen bonding to proteins that disrupts cellular membranes. Decreased membranolytic activity is associated with a lower silanol population and hence delayed macrophage death and consequent prolonged particle retention. Hydrophobic particles (less membranolytic) will be retained within the macrophages resulting in a continued source of stimulation and mechanism for transport within lung compartments and to extra-pulmonary lymphoid tissue.