Both human SP-A1 and Sp-A2 genes are expressed in small and large intestine.

The human SP-A locus consists of two functional genes and one pseudogene, and SP-A is shown to play a role in local host defense and the regulation of inflammation in lung. Because the intestine, like the lung, is constantly exposed to foreign and potentially harmful substances, we investigated the hypothesis that both human SP-A genes are expressed in intestine. We demonstrate that both SP-A genes are expressed in human small and large intestine. The presence of SP-A mRNA in human intestine was detected by reverse transcription polymerase chain reaction (RT-PCR), Northern blot analysis, and immunohistochemistry. The size of intestinal SP-A mRNA is the same as that in human lung, but the level of expression, compared with that in the lung, is very low in both the small and large intestine. Immunohistochemical analysis revealed positive reactivity for SP-A in a subgroup of epithelial cells in the intestine. Expression of both SP-A1 and SP-A2 genes was established by gene-specific PCR amplification, PCR-based converted RFLP discrimination, and direct sequencing of RT-PCR products. We speculate that SP-A in the intestine plays a role in local host defense and inflammation.

Surfactant regulation of host defense function in the lung: a question of balance.

Although the lung is protected by classic innate and adaptive immune mechanisms, another unique local immunoregulatory system involving pulmonary surfactant is described in this review. Normal surfactant inhibits many immune cell functions including proliferation resulting from various stimuli and production of reactive oxidants, inflammatory mediators, and some cell surface markers. The predominant surfactant lipids appear to be responsible for these suppressive effects. Conversely, surfactant proteins SP-A and SP-D stimulate many aspects of immune cell behavior. These proteins are collagenous lectins or collectins that bind to glycoconjugates on many pathogens, enhancing phagocytosis and killing in some cases. SP-A and SP-D stimulate chemotaxis and reactive oxidant generation, particularly in macrophages, although other cells are probably affected as well. In some cases, SP-A also stimulates the expression of cell surface markers and is involved in the stimulation of inflammatory mediators. Under normal conditions, the inhibitory effects of the lipid prevail, but the collectins may provide focal activation and stimulate immune cells at sites where they are needed. However, in some types of lung disease or after certain insults or exposures, the balance between these inhibitory and stimulatory influences may be disrupted and result in inflammatory injury.

Interaction of surfactant protein A with lipopolysaccharide and regulation of inflammatory cytokines in the THP-1 monocytic cell line.

Pulmonary surfactant protein A (SP-A) is involved in innate immunity in the lung. In this study we investigated the interaction of SP-A with different serotypes of lipopolysaccharide (LPS) on the regulation of inflammatory cytokines in vitro. In the human monocytic cell line, THP-1, combining SP-A with lipid A or rough LPS further enhanced lipid A- or rough LPS-stimulated tumor necrosis factor alpha (TNF-alpha) mRNA levels, while SP-A-elicited increases in TNF-alpha mRNA levels were partially neutralized. In contrast, the combination of smooth LPS and SP-A resulted in additive effects on TNF-alpha mRNA levels. We also demonstrated that there was cross-tolerance between SP-A and LPS in THP-1 cells. Pretreatment of THP-1 cells with LPS modestly inhibited the response of these cells to subsequent challenge with SP-A, with regard to the production of TNF-alpha, whereas there was no or little effect on the production of interleukin-1beta (IL-1beta) and IL-8. Conversely, pretreatment of THP-1 cells with SP-A markedly increased the response to subsequent challenge with LPS with regard to the production of IL-1beta and IL-8, although the production of TNF-alpha was modestly decreased. However, a synergistic stimulatory effect was observed when the two agents were added simultaneously to the cells. NF-kappaB formation was downregulated in SP-A- but not in LPS-induced tolerant cells. These results suggested that SP-A exhibits different interactions with distinct serotypes of LPS. In addition, SP-A is different from LPS with regard to the induction of cross-tolerance, and these actions may be mediated, at least in part, through different mechanisms.

Surfactant components modulate fibroblast apoptosis and type I collagen and collagenase-1 expression.

During lung injury, fibroblasts migrate into the alveolar spaces where they can be exposed to pulmonary surfactant. We examined the effects of Survanta and surfactant protein A (SP-A) on fibroblast growth and apoptosis and on type I collagen, collagenase-1, and tissue inhibitor of metalloproteinase (TIMP)-1 expression. Lung fibroblasts were treated with 100, 500, and 1,000 microg/ml of Survanta; 10, 50, and 100 microg/ml of SP-A; and 500 microg/ml of Survanta plus 50 microg/ml of SP-A. Growth rate was evaluated by a formazan-based chromogenic assay, apoptosis was evaluated by DNA end labeling and ELISA, and collagen, collagenase-1, and TIMP-1 were evaluated by Northern blotting. Survanta provoked fibroblast apoptosis, induced collagenase-1 expression, and decreased type I collagen affecting mRNA stability approximately 10-fold as assessed with the use of actinomycin D. Collagen synthesis and collagenase activity paralleled the gene expression results. SP-A increased collagen expression approximately 2-fold and had no effect on collagenase-1, TIMP-1, or growth rate. When fibroblasts were exposed to a combination of Survanta plus SP-A, the effects of Survanta were partially reversed. These findings suggest that surfactant lipids may protect against intraluminal fibrogenesis by inducing fibroblast apoptosis and decreasing collagen accumulation.

Comparison of SP-A and LPS effects on the THP-1 monocytic cell line.

Surfactant protein A (SP-A) increases production of proinflammatory cytokines by monocytic cells, including THP-1 cells, as does lipopolysaccharide (LPS). Herein we report differences in responses to these agents. First, polymyxin B inhibits the LPS response but not the SP-A response. Second, SP-A-induced increases in tumor necrosis factor-alpha (TNF-alpha), interleukin-1beta (IL-1beta), and IL-8 are reduced by >60% if SP-A is preincubated with Survanta (200 microgram/ml) for 15 min before addition to THP-1 cells. However, the LPS effects on TNF-alpha and IL-8 are inhibited by <20% and the effect on IL-1beta by <50%. Third, at Survanta levels of 1 mg/ml, SP-A-induced responses are reduced by >90%, and although the inhibitory effects on LPS action increase, they still do not reach those seen with SP-A. Finally, we tested whether SP-A could induce tolerance as LPS does. Pretreatment of THP-1 cells with LPS inhibits their response to subsequent LPS treatment 24 h later, including TNF-alpha, IL-1beta, and IL-8. Similar treatment with SP-A reduces TNF-alpha, but IL-1beta and IL-8 are further increased by the second treatment with SP-A rather than inhibited as with LPS. Thus, whereas both SP-A and LPS stimulate cytokine production, their mechanisms differ with respect to inhibition by surfactant lipids and in ability to induce tolerance.

Human SP-A protein variants derived from one or both genes stimulate TNF-alpha production in the THP-1 cell line.

In humans, two functional genes of surfactant protein (SP) A, SP-A1 and SP-A2, and several alleles of each functional gene have been characterized. SP-A is a multimeric molecule consisting of six trimers. Each trimer contains two SP-A1 molecules and one SP-A2 molecule. Until now, it has been unclear whether a single SP-A gene product is functional or whether there are functional differences either among alleles or between single-gene SP-A products and SP-A products derived from both genes. We tested the ability of in vitro expressed SP-A variants to stimulate tumor necrosis factor (TNF)-alpha production by THP-1 cells. We observed that 1) single-gene products and products derived from both genes stimulate TNF-alpha production, 2) there are differences among SP-A1 and SP-A2 alleles in their ability to stimulate TNF-alpha production, and 3) the increases in TNF-alpha production are lower after treatment with the SP-A1 alleles than after treatment with the SP-A2 alleles. Furthermore, coexpressed SP-As from SP-A1 and SP-A2 genes have a higher activity compared with SP-As from individual alleles or mixed SP-As from SP-A1 and SP-A2 genes. These data suggest that the SP-A-induced increases in TNF-alpha levels differ among SP-A variants and appear to be affected by SP-A genotype and whether SP-A is derived from one or both genes.

Surfactant protein-A reduces binding and phagocytosis of pneumocystis carinii by human alveolar macrophages in vitro.

Surfactant protein-A (SP-A) levels are increased in Pneumocystis carinii pneumonia, but the role of SP-A in the pathogenesis of P. carinii pneumonia is not completely understood. This study investigated the effect of SP-A on the in vitro binding and phagocytosis of P. carinii by normal human alveolar macrophages (AM). Determination of binding and phagocytosis was done with a fluorescence-based assay, utilizing fluorescein isothiocyanate (FITC)-labeled P. carinii. Binding and phagocytosis of P. carinii to AM correlated inversely with the levels of SP-A present on the surface of the organisms (r = -0.6323, P = 0.0086; and r = -0.9827, P < 0.0001, respectively). The addition of exogenous SP-A to organisms with low surface-associated SP-A reduced P. carinii binding by 30% (P < 0.05) and reduced phagocytosis by 20% (P < 0.05), whereas this effect was reversed with ethylenediamine tetraacetic acid (EDTA) or anti-SP-A antibody. Furthermore, binding and phagocytosis were enhanced after enzymatic removal of P. carinii surface-associated SP-A, and this effect was reversed with the addition of exogenous SP-A. The observed inhibitory effect of SP-A on P. carinii binding and phagocytosis reflected binding of SP-A to the organisms rather than a direct effect of SP-A on the macrophages. These data suggest that increased levels of SP-A may contribute to the pathogenesis of P. carinii pneumonia through binding to the surface of the organism and interfering with AM recognition of this opportunistic pulmonary pathogen.

Surfactant protein A activates NF-kappa B in the THP-1 monocytic cell line.

The expression of many genes for which products are involved in inflammation is controlled by the transcriptional regulator nuclear factor (NF)-kappa B. Because surfactant protein (SP) A is involved in local host defense in the lung and alters immune cell function by modulating the expression of proinflammatory cytokines as well as surface proteins involved in inflammation, we hypothesized that SP-A exerts its action, at least in part, via activation of NF-kappa B. We used gel shift assays to determine whether SP-A activated NF-kappa B in the THP-1 cell line, a human monocytic cell line. Activation of NF-kappa B in THP-1 cells by SP-A doses as low as 1 microgram/ml occurred within 30 min of SP-A treatment, peaked at 60 min, and then declined. This activation is inhibited by known inhibitors of NF-kappa B or by simultaneous treatment of the cells with surfactant lipids. Moreover, the NF-kappa B inhibitors blocked SP-A-dependent increases in tumor necrosis factor-alpha mRNA levels. These observations suggest a mechanism by which SP-A plays a role in the pathogenesis of some lung conditions and point to potential therapeutic measures that could be used to prevent SP-A induced inflammation in the lung.

Effect of SP-A and surfactant lipids on expression of cell surface markers in the THP-1 monocytic cell line.

Pulmonary surfactant and its lipid components inhibit cell proliferation and cytokine expression. Surfactant protein A (SP-A) can stimulate these same functions. We assessed the impact of SP-A and surfactant lipids on the expression of the cell surface markers, CD14, CD54 (intercellular adhesion molecule-1), and CD11b, by the human monocytic cell line THP-1 using fluorescent antibody staining and fluorescence-activated cell sorting. Under basal conditions CD14 and CD54 were undetectable, and CD11b was expressed at low levels. Incubation of the cells in 1,25(OH)2D3 alone, or with low doses of surfactant lipids, increased CD14, CD54, and CD11b. Expression was increased further by SP-A. However, the SP-A-induced increases in cell markers were blocked by simultaneous treatment with lipid. The results suggest that the ability of the macrophage to participate in an inflammatory response is enhanced by SP-A alone or by surfactant containing a higher than normal proportion of SP-A. They further suggest that the addition of lipids results in a phenotype less prone to initiate an inflammatory reaction.

Surfactant protein A regulates cytokine production in the monocytic cell line THP-1.

Surfactant lipids inhibit cytokine production by immune cells, and surfactant protein A (SP-A) stimulates it. By enzyme-linked immunosorbent assay and mRNA blotting, we studied proinflammatory cytokine production by the monocytic cell line THP-1. SP-A caused increases in tumor necrosis factor (TNF)-alpha within 1 h, peaking at 4 h and then declining. Interleukin (IL)-1 beta increased and stayed elevated for 24 h. SP-A stimulated IL-8 also, peaking at 4 h, rapidly declining, and peaking again at 24 h. SP-A-dependent changes were detected for IL-6, but at higher SP-A doses. mRNA levels for TNF-alpha and IL-1 beta increased in response to SP-A, peaking within 2 h. The increases in TNF-alpha mRNA and protein induced by SP-A were inhibited by surfactant lipids. For IL-1 beta and IL-8, the lipids either had no inhibitory influence or inhibited less than for TNF-alpha. This suggests that the ability of macrophages to participate in inflammatory reactions is enhanced by SP-A alone or by mixtures of lipids and SP-A containing more SP-A than in normal surfactant, as occurs in many conditions leading to inflammation.

Surfactant protein-A levels increase during Pneumocystis carinii pneumonia in the rat.

In bronchoalveolar lavage (BAL) of human immunodeficiency virus (HIV)-infected patients with Pneumocystis carinii pneumonia and in lungs of glucocorticoid-immunosuppressed rats infected with P. carinii, surfactant phospholipid levels are reduced. However, levels of the surfactant-associated protein-A (SP-A) in BAL are 4-5 times higher than normal in patients with P. carinii pneumonia. In this study, we examined the effects of glucocorticoid immunosuppression and P. carinii infection on SP-A messenger ribonucleic acid (mRNA) and protein levels in rat lungs. Rats were immunosuppressed by adding dexamethasone to their drinking water and were infected with P. carinii by intratracheal instillation of the organism. SP-A was measured by enzyme-linked immunosorbent assay (ELISA) and SP-A mRNA by hybridization of Northern blots with an SP-A complementary deoxyribonucleic acid (cDNA) probe. There was a severalfold increase in SP-A protein and mRNA levels in uninfected glucocorticoid-treated rats. However, contrary to what has been reported with the surfactant-associated lipids, SP-A mRNA and protein levels in P. carinii-infected animals were significantly higher than those found in the uninfected, immunosuppressed animals. Our results demonstrate that SP-A increases, probably as a result of elevated mRNA levels, in immunosuppressed rats with P. carinii infection and are consistent with our findings in HIV-positive patients with P. carinii pneumonia.

Changes in surfactant protein A mRNA levels in a rat model of insulin-treated diabetic pregnancy.

Maternal diabetes during pregnancy is associated with increased risk of neonatal respiratory distress syndrome (RDS). Previous studies using rat models for the diabetic pregnancy have documented decreased amounts of surfactant protein mRNA in the lungs of fetuses. In this study, we measured fetal lung surfactant-associated protein A (SP-A) mRNA from diabetic rats treated with insulin by daily injection or osmotic pump. Lungs were taken from fetuses on gestational d 20, and RNA was isolated and subjected to Northern blotting and densitometry to quantify SP-A mRNA. Fetal lung SP-A mRNA from untreated diabetic pregnancies was 34 +/- 2.9% of control. Insulin treatment increased levels to 55 +/- 4.2% of control values. Fetal lung SP-A mRNA levels were affected by the timing, length, and effectiveness of insulin treatment. Although levels from all treatment groups were still less than control values, insulin treatment during the last 5 or 10 d of pregnancy resulted in a substantial increase in SP-A mRNA levels over those of from untreated diabetic pregnancies. However, fetuses from the group with insulin treatment for the entire pregnancy showed decreases in fetal SP-A mRNA levels. Although the mechanism(s) responsible for the effects of diabetes and its treatment on fetal SP-A expression remain unclear, it appears unlikely that hyperglycemia is the principal cause.

Surfactant protein A stimulation of inflammatory cytokine and immunoglobulin production.

Pulmonary surfactant plays a variety of roles related to the regulation of immune function in the lung. Of particular interest in this regard is surfactant protein A (SP-A), a calcium-dependent lectin. We have reported previously that SP-A enhances concanavalin A-induced proliferation, and in this study we examined the secretion of tumor necrosis factor-alpha (TNF-alpha), interleukins 1 alpha, 1 beta, and 6, and interferon-gamma by human peripheral blood mononuclear cells. Levels of all of the cytokines except interferon-gamma were increased by SP-A. In rat peripheral blood cells, splenocytes, and alveolar macrophages we found a similar enhancement of TNF-alpha release by SP-A. In combinations of SP-A and surfactant lipids, the increased levels of TNF-alpha resulting from SP-A treatment decreased as the lipids increased. At higher relative concentrations of SP-A, the lipids had little or no effect. SP-A also enhanced the production of immunoglobulins A, G, and M by rat splenocytes. Levels of each isotype were increased severalfold over control levels. These data demonstrate that SP-A is capable of modulating immune cell function in the lung by regulating cytokine production and immunoglobulin secretion.

Effects of surfactant protein A and surfactant lipids on lymphocyte proliferation in vitro.

We studied the effects of dipalmitoyl L-alpha-phosphatidylcholine (DPPC), Survanta, surfactant protein A (SP-A), and mixtures of these substances on mitogen-induced lymphocyte proliferation using concanavalin A as a mitogen. A concentration-dependent suppression of proliferation was observed with 50-250 micrograms/ml of DPPC or Survanta. However, when SP-A was added to cultures, proliferation was stimulated. The inhibitory effects of DPPC and Survanta were altered in mixtures that contained SP-A. When added to 50 micrograms/ml of Survanta, SP-A reversed the inhibitory influence of Survanta and caused increased proliferation. These findings suggest that surfactant phospholipids cause a suppression of mitogen-induced lymphocyte proliferation, which is reversed somewhat by addition of SP-A. We hypothesize that immune cell function in the lung varies with changes in the relative amounts of surfactant components. Changes in surfactant composition may occur during pulmonary inflammation or infection or with surfactant replacement therapy and may influence immune and inflammatory processes in the lung.

Ultrastructure of lung in surfactant protein B deficiency.

Congenital alveolar proteinosis (CAP), a cause of respiratory failure in fill-term newborns, often leads to death in infancy despite medical therapy. We recently described an inherited deficiency of surfactant protein B (SP-B) (N. Engl. J. Med. 1993; 328:406-410) in two siblings with CAP. The SP-B deficiency was accompanied by marked abnormalities, both quantitative (increase) and qualitative (distribution), of SP-A and SP-C in the lungs of the affected infants. Ultrastructural studies of the lung of one of these infants and of a third affected sibling born in the index family showed abundant alveolar concentric multilamellated structures and membranous vesicles but no typical tubular myelin. In addition, membranous vesicles from type II cells and immunogold labeled SP-A and SP-C were found between type II cells and their basement membrane despite intact interepithelial cell junctions. These findings suggest an important role for SP-B in the directionality of surfactant secretion and in the formation of tubular myelin.

Secretion of surfactant protein A from rat type II pneumocytes.

Secretion of surfactant phosphatidylcholine has been extensively studied and there is evidence that it is a regulated process that can be influenced by a variety of physiological factors and pharmacological agents. In contrast, secretion of the major surfactant protein, surfactant protein A (SP-A), has been investigated to much lesser extent. It is not known whether SP-A secretion is constitutive or regulated and, if regulated, whether its regulation is similar to that of phosphatidylcholine. To address those questions we measured SP-A secretion in primary cultures of type II pneumocytes under conditions identical to those used to study phosphatidylcholine secretion. Freshly isolated cells from adult rats were cultured overnight, washed, and then incubated in fresh medium in the presence and absence of surfactant phospholipid secretagogues. As previously reported for phosphatidylcholine, SP-A secretion was linear with time for up to 4 h. However, the rate of SP-A secretion, approximately 6% of total SP-A (cells+medium) released into the medium per hour, was more than sixfold greater than that of the lipid. Although freshly isolated cells contained 70% more SP-A than cells that were cultured overnight, the rate of SP-A secretion was not significantly different. Secretion of SP-A by freshly isolated or cultured type II cells was not increased by a combination of ATP, terbutaline, the adenosine A2 receptor agonist 5'(N-ethylcarboxyamido)adenosine, 12-O-tetradecanoylphorbol-13-acetate, and ionomycin at concentrations that optimally stimulated phosphatidylcholine secretion. We conclude that secretion of the major lipid and protein components of surfactant are independently regulated.

Immunogold localization of SP-A in lungs of infants dying from respiratory distress syndrome.

Prematurely born infants can develop the neonatal respiratory distress syndrome (RDS) because of a deficiency of pulmonary surfactant. This lipoprotein complex synthesized by type II pneumocytes has different ultrastructural forms--intra- and extracellular lamellar bodies, which within the alveoli are transformed into tubular myelin, and this in turn gives rise to the surface monolayer, the functionally active form of surfactant. We have previously shown that at autopsy RDS lungs lack tubular myelin and have decreased immunoreactivity for antisera to surfactant protein A (SP-A), an important component of tubular myelin. Therefore, we proposed a role for SP-A in the conversion of lamellar bodies to tubular myelin and in the pathogenesis of RDS. To explore this possibility further, we compared in 14 RDS and 14 control lungs the distribution of SP-A in ultrathin sections, using affinity-purified rabbit anti-human-SP-A IgG and goat anti-rabbit IgG-conjugated with 10 nm colloidal gold particles. In controls, gold label was present in lamellar bodies, endoplasmic reticulum, on the cytoplasmic membrane of type II cells, and on lamellar bodies and tubular myelin either within alveoli or macrophages. In RDS lungs, reduced label was present in the same intracellular compartments and organelles, except in tubular myelin, which is absent. It is postulated that if SP-A is indeed necessary for the conversion of lamellar bodies to tubular myelin, in RDS either there is a deficiency of adequate amounts of functional SP-A or some other important component of surfactant is missing.

Delayed hydrophobic surfactant protein (SP-B, SP-C) expression in fetuses of streptozotocin-treated rats.

Tissues from fetuses and neonates of control and streptozotocin (STZ)-treated Sprague-Dawley rats were used to study the content and distribution of the hydrophobic surfactant protein B (SP-B) and the mRNAs for SP-B and SP-C using immunohistochemistry, RNA blotting, and tissue in situ hybridization. A dose of 50 mg/kg STZ was used to treat female rats before mating. The fetuses were sacrificed at fetal days 18 through 21 and neonates were obtained on neonatal days 1 and 2 (day of birth = end of day 22). At fetal day 18, SP-B was barely detectable by immunohistochemistry in control animals but the levels were progressively increased through gestation and easily detected by fetal day 21. At all fetal ages, SP-B was decreased in the STZ group compared with control animals. Both SP-B and SP-C mRNA were detectable at fetal day 18 in the control group and increased with advancing gestational age. In fetal lungs from the STZ group, SP-B and SP-C mRNA also showed an increase with advancing gestational age, but the levels were decreased compared with controls at fetal days 18, 20, and 21 (P less than 0.05). At fetal day 19, this difference did not achieve statistical significance. Differences between the two groups were no longer detected by neonatal days 1 and 2. The difference between the STZ and control groups, in both protein (SP-B) and mRNA (SP-B and SP-C), diminished with advancing fetal age but remained significant up to fetal day 21.(ABSTRACT TRUNCATED AT 250 WORDS)

Surfactant protein A expression is delayed in fetuses of streptozotocin-treated rats.

The content and distribution of the 26-to 38-kDa surfactant protein (SP-A) and its mRNA were determined in fetuses of control and streptozotocin (STZ)-treated Sprague-Dawley rats using immunohistochemistry, RNA blotting, and in situ hybridization. Female rats were treated with 50 mg/kg STZ before mating, and the fetuses were killed at fetal days 18-21 or on neonatal days 1 and 2 (day of birth = end of day 22). SP-A was barely detectable on fetal day 18 in controls and easily detected by fetal day 21. In the STZ group, SP-A was decreased compared with controls at fetal days 18-21. However, by neonatal days 1-2, there were no significant differences in SP-A levels between groups. SP-A mRNA was detectable at fetal day 18 in controls, but it was decreased in the STZ group at day 18-21 (P less than 0.02) and differences were no longer detected by neonatal days 1-2. SP-A and SP-A mRNA accumulated with advancing gestational age in both groups until neonatal days 1-2. The differences in SP-A and SP-A mRNA levels in the two groups diminished with advancing age but remained significant at fetal day 21. These data suggest that STZ-induced diabetes interferes with normal expression of SP-A in the developing fetal lung.