Epithelial Innate Immunity

The gut epithelium is an essential interface in insects that transmit parasites. We investigated the role that local innate immunity might have on vector competence, taking Stomoxys calcitrans as a model. S. calcitrans is sympatric with tsetse flies, feeds on many of the same vertebrate hosts, and is thus regularly exposed to the trypanosomes that cause African sleeping sickness and nagana. Despite this, S. calcitrans is not a cyclical vector of these trypanosomes. Trypanosomes develop exclusively in the lumen of digestive organs, and so epithelial immune mechanisms, and in particular antimicrobial peptides (AMPs), may be the prime determinants of the fate of an infection. To investigate why S. calcitrans is not a cyclical vector of trypanosomes, we have looked in its midgut for AMPs with trypanolytic activity. We have identified a new AMP of 42 amino acids, which we named stomoxyn, constitutively expressed and secreted exclusively in the anterior midgut of S. calcitrans. It displays an amphipathic helical structure and exhibits a broad activity spectrum affecting the growth of microorganisms. Interestingly, this AMP exhibits trypanolytic activity to Trypanosoma brucei rhodesiense. We argue that stomoxyn may help to explain why S. calcitrans is not a vector of trypanosomes causing African sleeping sickness and nagana.

Epithelial intestinal innate immunity plays a major role in the control of infectious diseases in vertebrates (1,2). In invertebrates, data are still fragmentary despite gut epithelium being an essential interface for parasites during their development in insect vectors. Understanding vector biology is a key element in the control of many parasitic diseases. In this context, the comparison of the trypanosome vector Glossina with the sympatric but non-vector Stomoxys calcitrans is particularly interesting. The tsetse fly Glossina spp. is the major vector of the range of trypanosomes that cause African sleeping sickness in humans and nagana in livestock. Stable flies, Stomoxys spp., feed on the same vertebrate hosts as tsetse flies and have a very similar digestive physiology and midgut anatomy. Although Stomoxys is constantly exposed to trypanosomes, it kills them in the midgut within 2-4 days of ingestion (3). So Stomoxys is not a cyclical vector of trypanosomes (although it can act as a mechanical vector, "flying pin"). Why Stomoxys is not a cyclical vector of trypanosomes is unknown. Recent studies have shown that the insect immune system plays a determinant role in the fate of trypanosome infections in tsetse flies (4,5). Consequently, in this report, we address the possibility that the distinction in vectorial capacity between Glossina and Stomoxys may lie in differences in immune mechanisms.
Trypanosoma vivax matures entirely in the mouthparts of Glossina. The other trypanosomes causing nagana and human disease are ingested into the fly midgut where they multiply first in the endoperitrophic space and later in the ectoperitrophic space tightly sandwiched between the peritrophic matrix and the anterior midgut epithelium (6). These trypanosomes then migrate, entirely within the lumenal space of the intestine, to the mouthparts or salivary glands for transmission to a new vertebrate. Consequently it is epithelial rather than systemic immune responses that are likely to have the major bearing on the fate of trypanosome infections. Epithelia constitute the first line of defense in the innate immunity of both vertebrates (reviewed in Ref. 7) and invertebrates (8 -10), and AMPs 1 are essential components of this epithelial immunity. For example, the mouse gut epithelium responds to bacterial infection by secretion of ␣-defensins or cryptdins (reviewed in Refs. 1 and 2), and gut expression of ␤-defensins is up-regulated by the parasite Cryptosporidium parvum (11). In invertebrates, the S. calcitrans anterior midgut epithelium responds to microbial infection by post-transcriptional up-regulation of secreted defensins, whereas the mosquito midgut responds to Plasmodium infection by up-regulation of a range of immunity factors including several AMP genes (reviewed in Ref. 12). We know that some of these AMPs can be active against trypanosomes (4) and other parasites such as Plasmodium and filarial worms that also spend part of their life in the gut of a bloodsucking insect (13,14).
Because of the potential importance of epithelial AMPs for determining vectorial capacity, we have looked for novel AMPs in gut extracts of S. calcitrans using biochemical and molecular approaches. It is already known that the anterior midgut epithelium of S. calcitrans produces two defensins (9) that are secreted into the midgut lumen (15,16). In this study, in addition to these defensins, we identified a novel 42-residue peptide, which we named stomoxyn. Stomoxyn has no sequence homology with other known proteins. This molecule is adult and anterior midgut-specific, where it is constitutively expressed. As shown by a circular dichroism study, stomoxyn adopts an ␣-helical structure only in the presence of an organic solvent that mimics a phospholipid membrane. Stomoxyn is active against many Gram-negative and Gram-positive bacteria, filamentous fungi, and yeast, and its activity is apparent within a few minutes of exposure. Although stomoxyn has only limited hemolytic activity against bovine red blood cells, it possesses significant trypanolytic effect on trypomastigote forms of Trypanosoma brucei rhodesiense. Consequently it may help to explain why Stomoxys is not a vector of African trypanosomiasis.

Insect and Tissue Preparation
S. calcitrans were reared as described by O'Brochta et al. (17). Adult male and female insects, both of which feed on blood, were used in the experiments. The artificial blood meal (18) and casein hydrolysate meal were made with high purity water. Flies used for HPLC purification of midgut AMPs were fed from cotton wool soaked swabs on either pig blood or casein hydrolysate (20 g/100 ml). At 24 -36 h after feeding, anterior midguts (2,000 consisting of proventriculus, thoracic, and reservoir regions) (19) were dissected in 154 mM NaCl and homogenized at 4°C in 200 mM sodium acetate at pH 4.5. The homogenate was heated to 100°C for 5 min and centrifuged at 12,000 ϫ g for 10 min at 4°C. Pellet and supernatant were stored separately at Ϫ20°C until used.

Isolation and Structural Characterization of AMPs
AMP Purification-The supernatant of S. calcitrans midgut homogenate was subjected to RP-HPLC on an Aquapore OD300 C 18 column (220 ϫ 4.6 mm, Brownlee TM ) with a linear gradient of 2-60% acetonitrile in acidified water (0.05% trifluoroacetic acid) over 120 min at a flow rate of 0.8 ml/min at 35°C. The column effluent was monitored by absorbance at 225 nm, and fractions were hand-collected. After evaporation under vacuum (Speedvac, Savant), fractions were reconstituted in MilliQ™ water (Millipore). An equivalent of 40 midguts/2 l was tested for antimicrobial activity using a solid growth inhibition zone assay. HPLC fractions with antimicrobial activity were further purified to homogeneity on a microbore Aquapore RP 300 C 8 column (1 ϫ 100 mm, Brownlee™) by appropriate linear biphasic gradients of acetonitrile in 0.05% trifluoroacetic acid over 60 min at a flow rate of 80 l/min. The column effluent was monitored by absorbance at 214 nm. Purity of the fractions with antimicrobial activity was controlled between each chromatography by matrix-assisted laser desorption ionization-time-offlight mass spectrometry (MALDI-TOF MS). Pure peptides were finally subjected to Edman degradation. All HPLC purifications were performed at 35°C with a Waters HPLC system equipped with a pump model 626, a controller model 600S, and a UV detector model 486.
MALDI-TOF MS-MALDI-TOF MS analysis was performed on a Bruker Daltonique (Bremen, Germany) BIFLEX III TM mass spectrometer in a positive linear mode with an external calibration. Samples were prepared according to the sandwich method (20) following the procedure reported previously (21).
Microsequence Analysis-Automated Edman degradation of the purified AMPs and detection of the phenylthiohydantoin derivatives were carried out using pulse liquid automatic sequencer (Applied Biosystems Inc., model 473A).

Library Construction, Screening, and Sequencing
A S. calcitrans adult midgut cDNA library with an estimated complexity of 1.4 ϫ 10 6 plaque-forming units was constructed in Lambda ZAP (Stratagene, La Jolla, CA) according to the manufacturer's instructions. The library was plated using Escherichia coli XL-1 Blue. A degenerate sense PCR primer (S1 5Ј-ATGACRGCNGTRTCYTT-3Ј) corresponding to the amino acid sequence KDTAVI of stomoxyn and an M13Ϫ20 primer were used in PCR with the cDNA library as template to generate a 336-bp product. This product was cloned using TOPO TA (Invitrogen) and sequenced. A pair of sequence-specific primers were designed from this sequence (S2, 5Ј-TGTATCCAATTCACTTTTCA-3Ј, and S3, 5Ј-TATGACAGCAGTATCCTT-3Ј) and used in PCR with the cDNA library as template to generate a 193-bp probe. The probe was 32 P-labeled and used to screen the midgut cDNA library. Phagemid clones (pBluescript) containing the stomoxyn protein cDNA were excised in vitro from the vector using ExAssist helper phage and plated using E. coli XLOLR (Stratagene) and sequenced.

Peptide Synthesis
The peptide was assembled using a multichannel peptide synthesizer adapted to Boc chemistry (22). Classical Boc-protected amino acids were used during the assembly but with histidine residues introduced as Boc-His(dinitrophenyl) derivatives. Following the last Boc deprotection, the resin was washed five times with dichloromethane and dried under nitrogen. Finally a standard fluorhydric acid procedure was used for deprotection and cleavage of the peptide from the resin. After removal of the fluorhydric acid by vacuum, the resin was washed with ethyl ether, and the peptide was extracted with acetic acid, water, and acetonitrile. The synthetic peptide was purified to homogeneity by solid-phase extraction and RP-HPLC using a gradient of acetonitrile in acidified water. Peptide purity and integrity were controlled by MALDI-TOF MS.

Bioassays
Solid Growth Inhibition Zone Assay-During the peptide purification, antimicrobial activity was assayed against the two test bacteria, the Gram-positive Micrococcus luteus (23) and the Gram-negative E. coli K12 RM148 (9). Briefly, 2 l (40 -230 midguts, depending on the purification step) of each RP-HPLC fractions were incubated with bacteria overnight at 37°C. Two AMPs were used as positive controls: Drosophila cecropin A (25 M) and Aedes defensin A (15 M) for anti-Gram-negative and -positive activities, respectively.
Liquid Growth Inhibition Assay-The activity spectrum (minimal inhibitory concentration, MIC) of stomoxyn (concentration range, from 0.2 up to 100 M) was determined against bacteria and fungi using a liquid growth inhibition assay (23). The strains used were from private and public collections (24,25). Bactericidal effect was measured by colony-forming unit counting at 24 h. When the antimicrobial activity was assayed in the presence of salts, the appropriate medium was supplemented with 1 mM CaCl 2 , 50 mM KCl, or 154 mM NaCl.
Kinetic of Killing against Bacteria and Yeast-Overnight cultures of E. coli K12 RM148, M. luteus, and Cryptococcus neoformans were diluted in appropriate media and allowed to grow to logarithmic phase. Stomoxyn (10 times the MIC) was incubated with the test microorganism in a 96-well microtiter plate at 37°C. At different time points (5,15,45, and 150 min), samples were removed, diluted, and plated on Luria Bertani broth or Sabouraud agar plate for colony-forming unit counting.
Kinetic of Killing against Filamentous Fungi-Fungal spores (10 4 / ml) of Neurospora crassa were incubated in Eppendorf tubes with 25 M stomoxyn (4 times the MIC value). At different time points (30, 60, 150, 270 min, and overnight), spores were washed twice with fresh potato dextrose broth (half-strength) and incubated for an additional 48 h. Fungal growth was measured at 550 nm.
Antitrypanosomal in Vitro Assay-The antitrypanosomal assay was performed using the Alamar Blue assay according to Raez et al. (26). Briefly, bloodstream forms of T. b. rhodesiense STIB 900 were added to sterile 96-well microtiter plates in 100 l of culture medium according to Baltz et al. (27) containing 300 bloodstream forms with or without a 3-fold serial drug dilution. The top concentration of stomoxyn was 500 g/ml (approx. 113 M). The test was run in duplicate. After 72 h of incubation, 10 l of Alamar Blue were added to each well, and the plates were incubated for another 2-4 h. The plates were read in a microplate fluorometer system (Spectramax Gemini, Molecular Devices) using an excitation wavelength of 536 nm and an emission wavelength of 588 nm. Data were transferred into a graphic program (Softmax Pro, Molecular Devices), sigmoidal inhibition curves were determined, and IC 50 (drug concentration inhibiting 50% of fluorescence development) values were calculated. The activity against procyclic forms was determined in the same way. Procyclics of the same trypanosome strain were grown in SDM-79 medium (28).
Hemolytic Assay-Bovine red blood cells were washed twice with phosphate buffer saline by centrifugation at 3,000 rpm, 10 min at 4°C until the supernatant was clear. A 10% suspension of bovine red blood cells was incubated in Eppendorf tubes in the presence of 2-fold dilutions of stomoxyn (0.2-100 M) for 1 h at 37°C. Samples (100 l) were centrifuged at 3,000 rpm for 5 min, and 90 l of supernatant were transferred in a microtiter plate. The absorbance of the supernatant was measured at 405 nm. As a positive control (100% lysis), bovine red blood cells were incubated with 0.1% SDS.
Serum and Trypsin Stability-The stability of stomoxyn (12 M final concentration) in the presence of 25% bovine serum was tested according to the procedure described previously (29) except that the stability was evaluated by MALDI-TOF MS. For the trypsin stability study, stomoxyn (0.5 mM in a final volume of 20 l) was incubated with bovine trypsin (Roche Molecular Biochemicals) at an enzyme/peptide ratio of 1/10, according to the manufacturer's directions. At different time points (5,15,30,60, 150 min, and overnight), an aliquot (0.5 l) of the sample was removed, and the reaction was stopped with trifluoroacetic acid. The digest products were analyzed by MALDI-TOF MS.
Circular Dichroism Analysis-Circular dichroism spectra of stomoxyn (0.1 mM) were recorded between 180 and 250 nm on a Jasco J-810 spectropolarimeter in 0.5 nm steps at varying concentrations of trifluoroethanol (TFE) in water (from 0 to 80%, v/v) at pH 4 using a quartz cell of 1 mm. For each experiment, four spectra were averaged, and the baseline was corrected for neat solvent and solvent mixture. All measurements were carried out at 25 and 4°C.

Stomoxyn Peptide Purification and Characterization
Crude homogenates of anterior midgut of S. calcitrans, 24 -36 h after a blood meal, have been shown previously to possess anti-Gram-negative activity. Two insect defensins have already been identified (Smd1 and Smd2 (9)). In this study, screening of HPLC-purified proteins extracted from 2,000 anterior midguts revealed the presence of eight fractions with antimicrobial activities. Two fractions were active against the Gram-positive test organism M. luteus (Fig. 1, peaks 1 and 2), and six were active against the Gram-negative strain E. coli K12 RM148 (Fig. 1, peaks 3-8). After MALDI-TOF MS measurement and partial Edman sequencing, fraction 8 was found to contain Stomoxys defensin Smd2 (data not shown). As the strongest activities were observed in fractions 5 and 7 (Fig. 1), an extensive purification was performed on these fractions, and pure active compounds with molecular masses of 4246.11 MH ϩ and 4415.49 MH ϩ were measured for fractions 5 and 7, respectively (data not shown). For the molecule at 4246.11 MH ϩ , the following 33-amino acid partial sequence was obtained, RG-FRKHFNKLVKKVKHTISETAHVAKDTAVIAG, whereas for the molecule at 4415.49 MH ϩ , a 30-amino acid partial sequence was obtained that was identical to the previous one. As searches in FASTA.GENOME and the SwissProt data base using the Blosum 50 scoring matrix did not establish significant similarity with already known peptides/proteins, we named this new molecule stomoxyn. The full-length stomoxyn sequence was obtained by screening of a midgut cDNA library. The sequence of the stomoxyn cDNA clone is 314 nucleotides long including 42 nucleotides of 5Ј-untranslated and 68 nucleotides of 3Ј-untranslated sequence flanking an open reading frame corresponding to 67 amino acid residues of the precursor protein (Fig. 2). According to the results of Edman degradation and cDNA cloning, the calculated molecular mass of mature stomoxyn is 4474.22 MH ϩ . The mass difference observed between this calculated mass and the mass measured for the active compound differed from 59 Da, corresponding to the lack of the C-terminal glycine residue and to amidation of the Cterminal threonine residue.

Expression of Stomoxyn mRNA in S. calcitrans
The expression of stomoxyn mRNA in different tissues and at various developmental stages in S. calcitrans was determined by nuclease protection assays (Fig. 3). Stomoxyn is expressed in the anterior midgut (proventriculus, thoracic, and reservoir regions) but not in the posterior midgut (opaque and lipoid regions) or the rest of the adult body, eggs, larvae, or pupae of S. calcitrans. The level of cDNA appeared to be unchanged with blood feeding, with and without addition of bacteria and laminarin. However, it should be noted that other immune peptides in these tissues are strongly regulated at the post-transcriptional level (16).

Activity Spectrum of Stomoxyn
Antimicrobial Activity-To investigate the antimicrobial properties of stomoxyn, the peptide was synthesized using Boc chemistry, purified to homogeneity, and tested against various microorganisms (Gram-negative and Gram-positive bacteria, filamentous fungi, and yeasts). The activity of stomoxyn was compared with a standard ␣-helical AMP from insects, Drosophila cecropin A. Stomoxyn had marked activity against most of the microorganisms tested (Table I). All the Gramnegative bacteria tested, except Enterobacter cloacae (6 M), were affected by less than 2 M peptide. No activity could be detected against the entomopathogenic Gram-negative bacteria, Serratia marcescens, up to 100 M. These activities are comparable with Drosophila cecropin A. Synthetic stomoxyn also showed high activity against most of the Gram-positive bacteria tested (MICs Ͻ 3 M), except for Bacillus subtilis (MIC ϭ 6.25-12.5 M). No activity was detected against Bacillus cereus, Listeria monocytogenes, or Staphylococcus aureus up to 100 M peptide. For all the Gram-positive bacteria tested, stomoxyn was more active than Drosophila cecropin A. In addition, stomoxyn was bactericidal on all the bacterial strains affected (data not shown). Most of the filamentous fungi assayed were highly sensitive to stomoxyn (MIC Ͻ 6.25 M), whereas only a few strains were affected by Drosophila cecropin A. The human pathogenic strain Aspergillus fumigatus was only moderately affected by 100 M stomoxyn, and Beauveria bassiana was unaffected, whatever the doses. However, A. fumigatus sporulation was inhibited by less than 6 M stomoxyn (Fig. 4A). In contrast to Drosophila cecropin A, which was inefficient on the four yeast strains tested, stomoxyn was highly active against C. neoformans (MIC ϭ 0.78 -1.56 M), moderately effective against Candida albicans and C. glabrata (MIC ϭ 25-50 M), and inefficient against Saccharomyces cerevisiae up to 100 M (Table I). Killing assays on the different pathogens (bacteria, filamentous fungi, and yeast) revealed that stomoxyn acted within the first few minutes of incubation (data not shown). The antimicrobial activity of stomoxyn was not affected by various concentrations of different salts such as KCl, CaCl 2 , or NaCl (data not shown).
Activity of Stomoxyn against Eukaryotic Cells-The antiparasitic activity of stomoxyn was tested against trypomastigote (vertebrate host forms) and procyclic (insect forms) forms of T. b. rhodesiense. The peptide was found to be lytic at 113 M with an inhibitory concentration (IC 50 ) of 37 M on trypomas-   (Fig. 4B). Against procyclic T. b. rhodesiense, the peptide was found to be inactive at the highest concentration tested (113 M; data not shown). Finally, as the anterior part of the midgut is a major site for blood storage and dehydration (30) but not hemolysis (31), stomoxyn was tested for its hemolytic activity. At 10 M, stomoxyn exhibited hemolytic activity of Ͻ 4% on bovine red blood cells, whereas at 100 M, hemolytic activity did not exceed 10% (Fig. 4C).

Stomoxyn Stability in Bovine Serum and to Trypsin
Because stomoxyn is present in the digestive tract where the blood meal is stored, we evaluated the stomoxyn resistance to bovine serum. To retard the kinetic of degradation and to increase peptide recovery, the stability experiment was performed in diluted serum (25%). After incubation of stomoxyn with bovine serum for 45 min at 37°C, MALDI-TOF MS did not reveal any significant cleavage product or reduction of the initial amount of native peptide (data not shown). However, after a 2-h incubation, a strong decrease in the amount of stomoxyn was observed through estimation based on the MALDI-TOF MS peak heights. When evaluating the first metabolites at 2 h, only C-terminal fragments of stomoxyn can be detected by MALDI-TOF MS, and no clear signal corresponding to the N-terminal part was detected. After 4 -5 h of incubation, no stomoxyn could be detected.
To assay the susceptibility of stomoxyn to trypsin, an enzyme present in abundance in the posterior midgut of the stable fly, a high concentration of peptide (0.5 mM) was incubated with pure trypsin, and the digest products were analyzed by MALDI-TOF MS in a time course experiment. After 5 min of incubation at 37°C, only traces of native stomoxyn were visible (Fig. 5). Precise mass spectrometric analyses of the crude digest revealed 12 tryptic fragments in the mass range of 400 Da to 3 kDa covering all the primary structure of stomoxyn (Fig.  5), confirming the extreme sensitivity of this peptide to trypsin.

Circular Dichroism Spectroscopy
The circular dichroism spectra of stomoxyn were acquired in increasing concentrations of TFE from 0 up to 80% at 4 and 25°C. In the absence of TFE and at 20% TFE, stomoxyn spectra are characteristic of a random coil conformation. As the TFE concentration increased, a shoulder at 222 nm and a shallow minimum in the vicinity of 208 nm appeared, indicating a transition from a flexible structure to a helical conforma-tion of the peptide in the presence of TFE. A marked effect of the temperature was visible at 30 and 40% TFE. At an equivalent concentration of TFE, the helicity was lower at 25°C (Fig.  6) than at 4°C (data not shown).

DISCUSSION
The present study reports the identification of stomoxyn, an AMP in the stable fly S. calcitrans with no homology with known proteins. Stomoxyn has no cysteine residue and adopts an ␣-helical structure in an organic solvent mimicking the phospholipidic membranes. As deduced from cDNA cloning, Edman degradation, and mass spectrometry measurement, stomoxyn is post-translationally modified by C-terminal amidation of the threonine residue. A truncated form of stomoxyn (molecule at 4256.11 MH ϩ ) lacking the two last C-terminal residues (Ala-Thr) was also isolated, probably corresponding to degradation by exopeptidases. We do not know whether this results from a natural degradation within the gut or from our experimental conditions. Alignment of the prepropeptide sequence of S. calcitrans stomoxyn and data base sequences revealed very little identity with any known peptide. The closest identity (p ϭ 0.44) was with a cecropin 3 precursor molecule (swiss protein database accession number Q94558) derived from Drosophila virilis, although little identity was found in the mature peptide. The greatest identity was between the first 5 amino acids of the prepropeptide domain of these molecules with the identical sequence, MNFYK. A second region of identity was found in the prepropeptide sequence KYLVVLVVLV-LCL, which showed some similarity (p ϭ 1.9) to the andropin precursor (accession number P21663) derived from Drosophila melanogaster (32).
As shown by nuclease protection assays, stomoxyn is specifically expressed in the anterior part of the midgut where the blood is stored. Blood is a highly valuable resource, which the fly must protect from microorganisms during storage prior to digestion. It is likely that stomoxyn, with its rapid acting broad range of activity against both bacteria and fungi (Table I), plays an important role in this protection. In previous studies on gut immunity of S. calcitrans, two defensins, Smd1 and Smd2, have been described that are also specific to the anterior midgut of adult flies (9,15). This production of at least three AMPs that are specific to adult midgut tissues illustrates the importance of the gut in the innate immunity of this blood-sucking insect and opens the possibility for the synergistic action of these various AMPs. Using the synthetic stomoxyn as reference, the quantity of native stomoxyn was estimated by -RP-HPLC to be ϳ530 pg/gut. We were not able to quantify the local concentration of stomoxyn. However, given that stomoxyn is likely to be secreted into the very confined space between the gel-like, dehydrated blood meal and the midgut epithelium, we speculate that the local concentration can reach a locally high FIG. 5. Stability of stomoxyn to trypsin. MALDI-TOF mass spectrum of the digest after 5 min of incubation at 37°C of stomoxyn (50 g) with trypsin (5 g). The labeled peaks (a-l) point out the identified tryptic fragments of stomoxyn. The labeled full lines above and below the primary structure of stomoxyn indicate the corresponding amino acid sequences. The asterisk stands for native stomoxyn. level as observed for vertebrate defensins (2). The synthesis of AMPs in the anterior part of the gut seems to be a conserved phenomenon in insects. Gambicin, cecropin, and defensin in Anopheles gambiae (8,21,24) and diptericin in D. melanogaster (10) are also secreted in the anterior part of the midgut. Digestive enzymes, secreted in the posterior part of the gut, may inactivate AMPs, as shown by the rapid degradation of stomoxyn by trypsin (Fig. 5).
Stomoxys is sympatric with Glossina, feeds on many of the same vertebrate hosts, and is thus regularly infected with trypanosomes but kills them efficiently in the gut (3). Glossina only feeds on vertebrate blood, whereas Stomoxys will feed on a variety of other materials in addition to blood including various sugar sources such as rotting fruit. As a consequence, the midgut of Stomoxys almost certainly receives a much stronger and more regular microbial challenge than that of the tsetse fly. So we might reasonably predict that the epithelial immune system of S. calcitrans is more highly developed than that found in the tsetse fly. The picture emerging to date is that the midgut of Glossina secretes defensin and diptericin using genes, which are also expressed in fat body (4). In contrast, S. calcitrans has at least three immune genes, which are specialized for exclusive use in the anterior midgut epithelium, two defensins (9) and stomoxyn. To control for the presence of the stomoxyn peptide in the tsetse genome, we carried out PCR experiments using two sets of stomoxyn primers and G. morsitans morsitans genomic DNA as template. Although stomoxyn was present in the S. calcitrans midgut cDNA library used, no such gene has been found in either G. m. morsitans genomic DNA or a G. m. morsitans midgut cDNA library (data not shown). In addition, stomoxyn shows an exceptionally wide spectrum of activities including trypanolytic activity on trypomastigote bloodstream forms of T. b. brucei. The fact that stomoxyn exhibited no antitrypanosomal effect against the procyclic stage indicates that the activity on the bloodstream forms must be fast before the first procyclic forms appear in the midgut, 24 -48 h after ingestion of the infected blood meal. Although trypanolysis occurs at a relatively high concentration on trypomastigote forms, such concentrations may well occur in vivo for the following reason. When a blood meal is taken, it strongly distends the peritrophic matrix, pushing it hard against the midgut epithelium. It is then rapidly dehydrated, giving the blood meal a gel-like consistency. Secretion of AMPs at this time is likely to lead to high local concentrations of AMPs in the very confined space between the anterior midgut epithelium and the gel-like blood meal, distending the peritrophic matrix. Interestingly, it is in this confined space that the developing trypanosomes concentrate. It is interesting to note that when ␣-defensins are secreted into the restricted space of the cryptal well in the vertebrate gut, local concentrations as high as a grams per liter range are estimated to occur (2). We suggest that the presence of stomoxyn in the anterior midgut of S. calcitrans may help to explain why Stomoxys is not a biological vector of trypanosomes.