Introduction: Secretory IgA (sIgA) is one of the principle antibodies in respiratory and other mucosal secretions. IgA antibody structure is critical for biological properties including the “noninflammatory properties” of antigen binding and interaction with other host defenses including polymorphonuclear (PMN) cells and other myeloid cells. In this regard, sIgA may protect the host from “collateral” damage due to exaggerated inflammatory responses to invasion by luminal pathogens. Loss of intact sIgA due to microbial cleavage may contribute to invasion and an exaggerated inflammatory response by the host. Pseudomonas aeruginosa (Ps) infection causes significant morbidity and mortality; a number of virulence factors are expressed by Ps which contribute to it’s pathogenicity. However sIgA cleavage by different Ps strains is not well established. The relative ability of different strains of Ps and other gram negative or gram positive bacteria to cleave sIgA and to modulate PMN inflammatory potential (IP) was studied in vitro.

Methods: Polyclonal sIgA was cocultured with a non-virulent strain of Ps (clinical strain, CS), a virulent strain of Ps (ExoT,U), Klebsiella pneumonia (Kp), Acinetobacter (Ac) or Staphylococcus aureus (Sa). Cleavage of sIgA was detected by measurement of intact sIgA and the cleaved secretory component (SC) by size exclusion and ELISA. In additional experiments PMNs were challenged with supernatants (sup) obtained from the above coculture and superoxide anion (O2-), elastase release and CD11b expression (mean fluorescent intensity, MFI) were measured. SIgA with PMNs served as control.

Results: Mean ± SD (N = 3 for each group)

Cleavage	Intact IgA(ng/ml)	SC(ng/ml)
Control	410.5 ± 10	21.8 ± 2
Kp	242.6 ± 6*	187.6 ± 3*
Ps(CS)	285.7 ± 4*	155.4 ± 3*
Ac	270.5 ± 7*	170.3 ± 5*
Sa	401.7 ± 8	72.1 ± 4
Ps(ExoT,U)	168.7 ± 3*	236.5±3*
PMN IP	Elastase (%)	O2-	CD11b(MFI)
Control	4.8 ± 0.3	3.4 ± 0.5	84.0 ± 3
Kp sup	19.3 ± 0.6*	19.0± 0.6*	141.5 ± 5*
Kp+IgA	8.9 ± 0.5$	9.1 ± 0.6$	95.4 ± 5$
Ps(CS) sup	19.3 ± 1*	20.2 ± 1*	148.9 ± 6*
CS+IgA	9.4 ± 1$	8.9 ± 1$	105.6 ± 5$
Ac sup	17.2 ± 1*	18.4 ± 1*	153.5 ± 6*
Ac+IgA	8.2 ± 0.4$	11.7± 0.5$	89.4 ± 4$
Sa sup	19.1 ± 2*	13.9 ± 2*	134.3 ± 7*
Sa+IgA	8.7 ± 1$	7.8 ± 1$	87.3 ± 4$
Ps(ExoT,U) sup	19.8 ± 2*	20.1 ± 2*	184.3 ± 6*
ExoT,U+IgA	18.6 ± 1	14.8 ± 2	178.3 ± 8

*p<0.001vs.control, $p<0.001vs.matched group (no IgA)

Conclusion: Intact sIgA decreased PMN inflammatory responses to all bacterial species. Cleavage of sIgA was noted with all gram negative bacteria; however cleavage was significantly greater with the virulent Ps strain. “Cleaved” sIgA was ineffective in modulating PMN responses to the virulent Ps strain. Loss of effective sIgA antibody function may be critical in the establishment of pneumonia and poorer outcome due to pneumonia from virulent Ps strains or other bacteria with significant IgA cleavage potential.