Large quantity of ribosomal RNA exists extracellularly in mouse spleen

When BALB/c mouse spleens were gently homogenized in saline, the resultant supernatant (without cells and tissue debris) contained significant amount of 28S and 18S ribosomal RNA, reaching up to 70% of the total spleen RNA. Haemoglobin assays indicated that less than 15% of the spleen cells were lysed during the homogenization process, indicating that the majority of the spleen 'supernatant RNA' was from the extracellular space of the organ rather than released by the splenocytes as a consequence of grinding. Quantitative RNA analysis showed that the ratio of spleen supernatant RNA/total RNA of BALB/c mice was inversely correlated with age (from approximately 70% at 3 weeks to 45% at 6 months), but that of BXSB mice (an animal model for systemic lupus erythematosus) remained at about 70% irrespective of age. Methyl Green-Pyronin Y staining of paraffin sections of mouse spleen revealed that extracellular RNA was distributed mainly in the sinuses of the organ. Culture supernatants of apoptotic splenocytes contained significant amounts of RNA, suggesting that the extracellular RNA in the spleen might have come from apoptotic lymphocytes. This is supported by the fact that 'thymus supernatant' also contained significant amount of RNA. A possible correlation between spleen extracellular RNA and autoimmune diseases is discussed.