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Cyclophosphamide-induced neutropenia exacerbates septic shock and multiple organ injury in conscious rats during Escherichia coli (EC) bacteremia despite antibiotics and fluid administration. We hypothesized that such shock and inflammatory organ injury would be mitigated by rBPI23's microbicidal activity and/or binding of EC endotoxins. Four days after 100 mg cyclophosphamide/kg, catheterized rats with < 300 PMNs/microL were pretreated with rBPI23 or the irrelevant 22 kDa protein thaumatin [3.3-6.6 mg/kg, i.v. in 0.9% NaCl (NS)] 5 min before graded i.v. infection with 5 x 10(9) or 1 x 10(10) cfu of EC serotype 055:B5 ending at t = 0. Posttreatment with each protein continued (3.3-6.6 mg/kg in 1 mL NS/h) through 8 h, in addition to penicillin plus amikacin sulfate at t = 1.5 and 8 h. Arterial samples were obtained before pretreatment and at t = 1.5, 4.5, 8, and 24 h when animals were necropsied. One of eight thaumatin + 5 x 10(9) EC rats and none of six thaumatin + 10(10) EC rats survived 24 h. In contrast, rBPI23 significantly reduced mortality after either inoculum, improved bacterial clearance, and led to renormalization of early EC-induced hypotension, hypothermia, tachypnea, hyperoxemia, and hypocarbia. Compared with thaumatin, however, rBPI23 did not reduce circulating endotoxin or bioactive and antigenic tumor necrosis factor-alpha. Sepsis-induced severe neutropenia (< 50 PMNs/microL) evident in all EC rats by t = 1.5 h was reversed with rBPI23 by t = 8 h, but thrombocytopenia (< 5 x 10(4) platelets/microL) evident in all groups by t = 4.5 h was not altered.(ABSTRACT TRUNCATED AT 250 WORDS)