Correlation of Pasteurella haemolytica leukotoxin binding with susceptibility to intoxication of lymphoid cells from various species

Pasteurella haemolytica, the causative agent of shipping fever pneumonia in cattle, produces a leukotoxin (LKT) which lyses ruminant leukocytes with h igh efficiency but is reputed to not affect leukocytes from non-ruminant sp ecies. In this study, we tested the supposition that LKT binding correlates positively,with susceptibility to intoxication of susceptible isolated bov ine lymphocytes and lymphoma tissue culture cells (BL3 cells) and negativel y with reputed nonsusceptible equine, porcine, and canine lymphocytes and h uman lymphoid tissue culture cells (Raji cells). Bovine lymphocytes and BL3 cells were highly susceptible to LKT intoxication, exhibiting bath substan tial increase in intracellular Ca2+ concentration and marked leukolysis. Ex posure of reputed LKT-nonsusceptible porcine lymphocytes and Raji cells to LKT caused a slightly increased intracellular Ca2+ concentration but no leu kolysis. No LKT effect was detected for equine and canine lymphocytes. LKT bound to lymphoid cells from all species tested. Intact 102-kDa LKT was rec overed from exposed isolated lymphoid cell membranes. Pro-LKT acylation aas not required for LKT binding to BW cells. LKT binding was rapid with maxim al binding occurring by 3 min, and was proportional to the LKT concentratio n in the range 0.04 to 4.0 mu g/ml. For this LKT concentration range, BL3 c ells bound more LKT than did porcine lymphocytes or Raji cells, suggesting that LKT binds to BL3 cells with higher affinity than to porcine lymphocyte s or Raji cells. Above 4.0 mu g/ml, LKT demonstrated saturable binding to B L3 cells. Neutralizing anti-LKT monoclonal antibody (MAb) MM601 diminished LKT binding to BL3 by 36% while decreasing leukolysis by 81%, In contrast, MM601 did not diminish LKT binding to Raji cells, Pretreatment of target ce lls with 120 mu g of protease K per mi diminished LKT binding to BL3 cells bg 75%, with only a 25% decrease in leukolysis, However, pretreatment with 150 mu g of protease K per mi abolished the remaining 25% of LKT binding an d 75% leukolysis. Therefore, P, haemolytica LKT binds rapidly to susceptibl e and to reputed nonsusceptible lymphoid cells, LKT binding resulting in sp ecies-specific leukolysis uas characterized by high affinity, inhibition by MAb MM601, and relative resistance to protease K pretreatment of lymphoid cells. Two types of LKT binding to lymphoid cells are proposed, High-affini ty binding leads to efficient leukolysis, In some lymphoid cells from reput ed LKT-nonsusceptible species low-affinity LKT binding may cause a low-effi ciency increase in the intracellular Ca2+ concentration without Leading to leukolysis.