A new virus of minimal pathogenicity associated with friend virus. II. Histological changes and immunodepressive effect

Histological reactions induced in BALB/c mice by a new virus with minimal pathogenic action — the Rowson‐Parr virus (RPV) — are described. The changes were virtually confined to the spleen and consisted of moderate splenomegaly (200–350 mg) associated with hyperplasia of the red pulp, particularly in the perifollicular zones, and enlargement and activation of the germinal centres. No proliferation of plasma cells was noted at any time. These changes persisted (with minor fluctuations) for 6 months or more, but are described in detail for only the first 3 weeks in the present account. A severe depression of the immune response to sheep erythrocytes was observed between the 2nd and 6th days after infection; a more moderate immunodepression was detected up to 6 months or more after infection. The significance of these findings is discussed.