TLR4-Independent upregulation of activation markers in mouse B lymphocytes infected by HRSV

Human respiratory syncytial virus (HRSV) is the most common cause of severe respiratory infections in infants and young children, often leading to hospitalization. In addition, HRSV poses a serious health risk in immunocompromised individuals and the elderly. It has been reported that this virus can infect mouse antigen-presenting cells, including B lymphocytes. In these B cells, HRSV infection upregulates the expression of activation markers, including MHC class II and CD86, but not MHC class I molecules. Here, we report that HRSV infection of spleen B lymphocytes downregulated TLR4. Either blocking with anti-TLR4 antibody or genetic deletion, but not functional deficiency of TLR4, moderately reduced the infectivity of HRSV in B lymphocytes. HRSV-infected B lymphocytes with deleted TLR4 upregulated MHC class II and CD86 molecules to the same levels as TLR4+ wild type B cells. Since the activation of monocytes and macrophages by HRSV was previously reported to depend on TLR4, the current study indicates that these cells and B lymphocytes respond to HRSV infection with different activation pathways.