Alleviation of immunosuppression in vitro by recombinant platelet factor 4

To elucidate the mechanism by which platelet factor 4 (PF4), a secreted platelet protein, and its C-terminal peptides alleviate suppression of the antibody response in vivo, their Immunoregulatory activity was studied In vitro, using cultured spleen cells from BALB/c mice primed with sheep red blood cells (SRBC). When addition of 48 h cultured concanavalln A (Con A) blasts at 5×105/ml significantly suppressed the antl-SRBC plaque-forming cell response, suppression was alleviated in 25 of 29 experiments by 0.2μg/ml recombinant (r)PF4 (6.4 nM if rPF4 is a tetramer). The effect of Con A blasts was abolished by clmetldine, a histamine type 2 (H2) antagonist. Dimaprit, an H2 agonist, at 1 –2×10−4 M, or splenic T cells that had been incubated for 1 h with dimaprit and washed, also caused significant suppression that was alleviated by 0.2 μg/ml rPF4 (n = 8), and by 0.02 μg/ml in six of these tests. The C-terminal 13 amino acid peptide of PF4 was active at 0.02–0.2 μg/ml (0.01 –0.11 μM), but peptides from the middle or N-terminal end of the molecule, or IL-8, which shares structural homology with PF4, were inactive. IL-1 and IL-2 raised control responses without affecting suppression or Its alleviation by rPF4. Neutralizing antibody to transforming growth factor β1 (TGF-β1) did not affect Con A blast-induced suppression and suppression Induced by exogenous TGF-β1 (0.5 ng/ml) was not counteracted by rPF4. Blocking prostaglandin production with 0.2 or 2 μM Indomethacln did not affect suppression significantly but reduced rPF4 activity; prostaglandin D2 restored the effect of rPF4. Thus, rPF4 and Its C-terminal portion counteract suppression Induced by H2 receptor stimulation In vitro and this activity of PF4 may need prostaglandin production.