Antibiotic therapy–induced collateral damage: IgA takes center stage in pulmonary host defense Juergen Lohmeyer, … , Rory E. Morty, Susanne Herold J Clin Invest. 2018;128(8):3234-3236. https://doi.org/10.1172/JCI122032. Commentary The use of broad-spectrum antibiotics in empirical antimicrobial therapy is a lifesaving strategy for patients in intensive care. At the same time, antibiotics dramatically increase the risk for nosocomial infections, such as hospital‑acquired pneumonia caused by Pseudomonas aeruginosa, and other antibiotic-resistant bacteria. In this issue of theJ CI, Robak and colleagues identified a mechanism by which depletion of resident gut and lung microbiota by antibiotic treatment results in secondary IgA deficiency and impaired anti–P. aeruginosa host defense. Impaired defenses could be improved by substitution of polyclonal IgA via the intranasal route in a mouse model of pneumonia. Importantly, antibiotic treatment caused lung IgA deficiency that involved reduced TLR-dependent production of a proliferation-inducing ligand (APRIL) and B cell–activating factor (BAFF) in intensive care unit patients. These patients might therefore benefit from future strategies to increase pulmonary IgA levels. Find the latest version: https://jci.me/122032/pdf COMMENTARY The Journal of Clinical Investigation Antibiotic therapy–induced collateral damage: IgA takes center stage in pulmonary host defense Juergen Lohmeyer,1 Rory E. Morty,1,2 and Susanne Herold1,3 1Department of Internal Medicine II, University of Giessen and Marburg Lung Center (UGMLC), member of the German Center for Lung Research (DZL), Justus-Liebig-University, Giessen, Germany. 2Department of Lung Development and Remodelling, Max Planck Institute for Heart and Lung Research, member of the German Center for Lung Research (DZL), Bad Nauheim, Germany. 3Division of Pulmonary and Critical Care, Northwestern University Feinberg School of Medicine, Chicago, Illinois, USA. biota changes induced by ABx treatment inhibit immunoglobulin A (IgA) produc- The use of broad-spectrum antibiotics in empirical antimicrobial therapy tion in the lung, thereby increasing suscep- is a lifesaving strategy for patients in intensive care. At the same time, tibility of ABx-treated patients to infection antibiotics dramatically increase the risk for nosocomial infections, such with P. aeruginosa (Figure 1). as hospital-acquired pneumonia caused by Pseudomonas aeruginosa, and other antibiotic-resistant bacteria. In this issue of the JCI, Robak and Microbiota-host immunity colleagues identified a mechanism by which depletion of resident gut and cross-talk lung microbiota by antibiotic treatment results in secondary IgA deficiency How do the findings of Robak et al. help and impaired anti–P. aeruginosa host defense. Impaired defenses could expand our understanding of the interplay be improved by substitution of polyclonal IgA via the intranasal route in between the microbiota and host defense a mouse model of pneumonia. Importantly, antibiotic treatment caused in lung infection, and thereby promote lung IgA deficiency that involved reduced TLR-dependent production of a development of new prophylactic or thera- proliferation-inducing ligand (APRIL) and B cell–activating factor (BAFF) in peutic strategies? It is well established that intensive care unit patients. These patients might therefore benefit from antimicrobial therapies inevitably cause future strategies to increase pulmonary IgA levels. dramatic and long-lasting collateral dam- age to the diverse populations of com- mensal bacterial species that are part of a patient’s intestinal microbiota. However, Antibiotic use and risk of risk factor for the normal microbiome to recent investigations have further revealed nosocomial infection turn into a so-called pathobiome, which is that composition of the microbiota also Broad-spectrum antibiotics (ABx) form diagnosed as hospital-acquired pneumo- crucially impacts systemic and pulmonary the backbone of empiric antimicrobial nia (HAP) and often associated with out- innate immune responses during bacterial treatment regimens, which are lifesav- growth of P. aeruginosa or other antibiotic- and viral infections (13–15). Considerably ing for those with severe infections such resistant bacteria (6, 7). Recent estimates less is known about how the microbiota as pneumonia, peritonitis, or sepsis, and indicate that in Europe, 2.6 million cases ecosystem and its depletion by ABx impact which constitute an integral part of treat- of hospital-acquired infections occur each adaptive immune responses. The gut ment guidelines (1). However, apart from year, and that HAP accounts for the high- microbiota is critical for regulating intesti- driving the selection of multiresistant est disease burden and number of deaths nal IgA production, as IgA-secreting cells pathogens, ABx severely disrupt the host (8). HAP dramatically increases both the and IgA production are almost absent in microbiome, which has been increasingly length of the hospital stay and health care the gut of germ-free mice (16). Moreover, recognized as a critical factor in shaping costs, and is associated with a mortality of microbial signals are known to activate host immunity (2). Antibiotic stewardship up to 13% (9). TLRs on intestinal epithelial cells and DCs and improved point-of-care microbial The increased susceptibility of critical- to induce production of the crucial B cell diagnostics increasingly facilitate the opti- ly ill patients to infection with multidrug survival signals APRIL and BAFF, which mized use of specific antibiotics (3), but in resistant (MDR) gram-negative bacteria in turn, promote IgA production by plas- critically ill patients the empiric use of ABx was originally attributed to generation of ma cells (17–19). In addition, microbiota- will remain indispensable. Point-preva- a permissive niche with selection of MDR derived short-chain fatty acids have lence studies have documented that to pathogens that were not covered by the recently been demonstrated to positively date 64%–71% of all patients in intensive ABx regimen (10, 11). In this issue, Robak regulate IgA production (20). care units (ICUs) receive antibiotics (4, 5). and colleagues (12) identify an additional Previous antimicrobial therapy is a major mechanism by which lung and gut micro- IgA-mediated pulmonary host defense Related Article: p. 3535 What makes IgA so special for lung host defense? Immunoglobulin A has a criti- Conflict of interest: The authors have declared that no conflict of interest exists. cal role in immune defense, particularly Reference information: J Clin Invest. 2018;128(8):3234–3236. https://doi.org/10.1172/JCI122032. at mucosal surfaces, and IgA is specially 3234 jci.org Volume 128 Number 8 August 2018 The Journal of Clinical Investigation COMMENTARY Figure 1. Broad-spectrum antibiotic treatment exerts severe collateral damage by inhibiting microbiota-induced secretory IgA synthesis and IgA- dependent lung host defense toward P. aeruginosa. The lung and gut commensal microbiota induce IgA production at mucosal surfaces involving TLR-, APRIL-, and BAFF-dependent signaling. Lung secretory IgA (sIgA) binds P. aeruginosa and reduces host susceptibility to P. aeruginosa pneumonia. ABx treatment destroys luminal microbiota and severely reduces sIgA production by lung IgA-secreting plasma cells, thereby impairing anti-pseudomonas host defense, which can be reestablished by transnasal administration of sIgA. equipped to undertake this task through had not been investigated if and how dis- However, it is also conceivable that the the unique structural attributes of the turbances to this mechanism contribute to induction of IgA generation occurs in the IgA heavy chain and by virtue of its abil- the enhanced susceptibility to lung infec- intestinal tract and that IgA+ plasma cells ity to polymerize (16). Notably, more IgA tions following antibiotic treatment. The subsequently migrate to the lung, given antibodies are synthesized in mammals findings by Robak et al. explain how deple- that migration of plasma cells between per day than antibodies of all other iso- tion of the resident microbiota affects lung lung and intestine has been previously types combined. IgA plays a pivotal role host defense against P. aeruginosa and described (22). Second, the cellular com- in maintaining a homeostatic relation- represent a fundamental step in filling this partment responsible for the TLR-depen- ship between the host and the resident knowledge gap. To our knowledge, this is dent APRIL production and the specific microbiota of the intestine (21). Produc- the first study to demonstrate that deple- TLR member involved remain unknown. tion of high-affinity and antigen-specific tion of resident microbiota by ABx inhib- Third, the mechanism by which IgA exerts IgA in Peyer’s patches and mesenteric its TLR-dependent production of APRIL, antimicrobial properties directed against lymph nodes has been documented to be resulting in secondary IgA deficiency in P. aeruginosa (and most likely other bac- T cell dependent (16), whereas low-affin- the lungs of both mice and ICU patients terial species) in the lung is unknown, ity polyreactive IgA responses are mainly and increases susceptibility to subsequent but likely is similar to the gut, where IgA induced in isolated lymphoid follicles P. aeruginosa infection (Figure 1). There achieves neutralization of invasive patho- and in subepithelial B cells in a T cell– are several essential unresolved questions