Critical splenic mass for survival from experimental pneumococcemia

Survival after intravenous challenge with live Streptococcus pneumoniae (type III) over a wide dose range (10², 10³, 10⁴, 10⁵, 10⁶, 10⁷) was examined in 686 Sprague-Dawley rats. These included Sprague-Dawley rats with varying splenic mass (25 to 300 mg) after graded partial amputation of the spleen (graded hyposplenia) (425 rats), total splenectomy (asplenia) (145 rats), and sham operation (eusplenia) (116 rats). Large (300 mg) but not small (< 100 mg) splenic remnants showed protection greater than asplenia (P < 0.001) and comparable to eusplenia while remnants of intermediate size (200 mg) showed intermediate protection-that is, greater than asplenia (P < 0.02) but less than eusplenia (P < 0.02). Moreover, the greater the spleen mass, the higher the LD₅₀-thus 2.2 × 10³ for no spleen; 3.3 × 10³ for 25 mg; 8.1 × 10³ for 100 mg; 5.2 × 10⁴ for 200 mg; 1.8 × 10⁵ for 300 mg and 9.9 × 10⁵ for whole spleen. Apparently, a critical mass of spleen (more than one-third) is needed to restore host resistance to fatal blood-borne infection from this encapsulated microorganism.