Effect of the B Subunit of the Cholera Toxin on the Raw 264.7 Murine Macrophage-Like Cell Line

The 125 I-labeled B-subunit of the cholera toxin ([ 125 I]CT-B with specific activity 98 Ci/mmol) was found to be bonded to the murine macrophage-like cells of the RAW 264.7 line with high affinity ( K d 2.3 nM). The binding of the 125 I-labeled CT-B was inhibited by the unlabeled interferon-α 2 (IFN-α 2 ), thymosin-α 1 , (TM-α 1 ), and the LKEKK synthetic peptide corresponding to the 16–20 sequence of human TM-α 1 and 131–135 sequence of human IFN-α 2 ( K i 0.9, 1.1, and 1.4 nM, respectively), but the KKEKL unlabeled synthetic peptide with the inverted sequence did not inhibit binding ( K i > 1 μM). In the concentration range from 10 to 1000 nM, CT-B and the LKEKK peptide dose-dependently increased the nitric oxide (NO) production by the cells, the activity of intracellular soluble guanylate cyclase (sGC), as well as the ability of the cells for adhesion, spreading, and digestion of bacteria of the 415 virulent strain of the Salmonella typhimurium in vitro. The KKEKL peptide was simultaneously tested and proved to be inactive. Thus, the binding of CT-B and the LKEKK peptide to the receptor on the RAW 264.7 cells resulted in an increase in their NO-synthase, guanylate-cyclase and phagocytic activity.