Medzhitov R, Janeway C A., Jr Innate immunity: impact on the adaptive immune response. whereas gram-negative bacteria preferentially stimulated secretion of IL-10 (650 versus 200 pg/ml; 0.001). Gram-positive species also induced stronger major histocompatibility complex class II-restricted IFN- production in unfractionated blood mononuclear cells than did gram-negative species (12,000 versus 3,600 pg/ml; 0.001). The poor IL-12-inducing capacity of gram-negative bacteria was not remediated by addition of blocking anti-IL-10 antibodies to the cultures. No isolated bacterial component could be identified that mimicked the potent induction of IL-12 by whole gram-positive bacteria, whereas purified LPS induced IL-10. The results suggest 3′-Azido-3′-deoxy-beta-L-uridine that gram-positive bacteria induce a cytokine pattern that promotes Th1 effector functions. The innate immune system is an ancient defense system found in all multicellular organisms. It is comprised of cells and proteins which are able to recognize molecular patterns common to large groups of microorganisms (32). Recognition of 3′-Azido-3′-deoxy-beta-L-uridine such danger signals results in the activation of various types of effector functions that eliminate or wall off the microorganism, such as phagocytosis, mucus secretion, complement attack, coagulation, etc. ZAP70 (31). Another important function of the innate immune system is to activate the more recently developed acquired immune system and to focus its attack on potentially dangerous antigens. T cells recognize their specific antigens in the form of peptides presented on major histocompatibility complex (MHC) class II molecules on the surface of antigen-presenting cells, such as monocytes, dendritic cells, or Langerhans cells. Antigen-presenting cells react to bacterial stimulation by secretion of T-cell-activating cytokines and expression of membrane-bound costimulatory molecules which bind to corresponding receptors on T cells (9). Only if T cells receive such positive signals concomitantly with stimulation via their antigen-specific receptor will they become activated and an immune response be initiated (36). Thus, microbial products function as adjuvants which augment specific immune responses. In this way, the broad specificity and immediate action afforded by the ancient innate immune system may be combined with the infinite receptor repertoire and versatility of acquired immunity. Two key cytokines that bridge the gap between innate and acquired immunity are interleukin 10 (IL-10) and IL-12. Both are produced by monocytes, macrophages, and dendritic cells in response to microbes (11, 18, 24), but they have largely opposite properties. IL-12 is a T-cell stimulatory cytokine which activates T cells and NK cells to secrete gamma interferon (IFN-) and to lyse target cells (40). T cells that are influenced by IL-12 during antigen presentation will mature into IFN–producing cells (17, 39). IL-10, in contrast, downregulates T-cell cytotoxicity and IL-12 and IFN- production and decreases presentation of antigens for T cells (10, 13, 14). Instead, IL-10 stimulates B-cell maturation and antibody production (35). We had previously observed that lactobacilli isolated from the human gastrointestinal mucosa, which are gram-positive 3′-Azido-3′-deoxy-beta-L-uridine commensal bacteria, induced secretion of large amounts of bioactive IL-12 from human monocytes, while gram-negative induced very little IL-12, but more IL-10 (23). Along the same lines, pneumococci, which are gram-positive respiratory pathogens and commensals, triggered more IL-12 production from human mononuclear cells than gram-negative instead induced more IL-10 (2, 3). A similar discrepancy between a gram-positive bacterium and a gram-negative bacterium was also 3′-Azido-3′-deoxy-beta-L-uridine noted for the two oral pathogens and (25). This prompted us to investigate whether gram-positive and gram-negative bacteria differ in their propensity to induce the partly opposite immunoregulatory cytokines IL-12 and IL-10. MATERIALS AND METHODS Bacteria and bacterial components. Gram-positive and gram-negative bacterial species of types inhabiting human respiratory or gastrointestinal mucosa were obtained from the Culture Collection of the University of G?teborg (CCUG; G?teborg, Sweden). They represented both clinical and commensal isolates (Table ?(Table1).1). A strain was isolated from rectal mucosa of a healthy human volunteer; this species represents the major lactobacillus group colonizing the human gastrointestinal tract (1). In addition, one strain each of sp., sp., sp. isolated from the feces of healthy breast-fed Swedish infants were included. These isolates had been passaged no more than two times and were stored frozen at ?70C before being used in the study. TABLE 1 Cytokine response of human monocytes to various gram-positive and gram-negative bacteriaa (18363)Adult intestineAnaerobic4,300??810160??2827 ?(1795)BovineAnaerobic160??4120??131.3 (541)Erythrasma, trunkAerobic2,000??340390??995.1 ?(19916)UnknownAerobic3,500??660200??4718 ?(67b)Rectum, healthyAerobic2,200??540160??5014 ?(1800)Pleural fluidAerobic4,000??850240??8317 ?(31611)Oral cavityAerobic3,600??490200??4418 Gram negative ?(4940)UnknownAnaerobic32??2350??390.09 ?(24)Urine, cystitisAerobic140??601,200??2800.12 ?(21594)Pharynx, healthyAerobic90??23560??1000.16 ?(12836)Septic fingerAerobic120??50650??1200.18 ?(23929)Pharynx, healthyAerobic460??160690??1500.67 ?(551)UnknownAerobic 30480??1200.06 ?(5123)Intestinal tractAnaerobic1,600??480770??2002.08 Open in a separate window aHuman monocytes from nine blood donors were purified by adherence and stimulated with each of 14 bacterial strains at a concentration of 5 106 UV-killed bacteria/ml. The bacteria were obtained from the CCUG, and their CCUG number given in parentheses. The strain was isolated from healthy human gastrointestinal mucosa (1). IL-12 p70, and.