Single domain antibodies: promising experimental and therapeutic tools in infection and immunity

Leukocytes express numerous ecto-enzymes that have their active sites exposed to the extracellular environment. These enzymes play important roles in cell trafficking, inflammation, and apoptosis [24, 25]. For example, NAD-dependent ADP-ribosyltransferases (ARTs, also named CD296) regulate the function of other cell surface proteins by post-translational modification [26, 27]. Other nucleotide-metabolizing ecto-enzymes (CD38, CD39, CD73, CD157, and CD203) control the half-life of purinoreceptor-ligands and/or generate signaling molecules, such as cyclic ADP-ribose, adenosine, and lysophosphatidic acid [28]. Peptidases (CD10, CD13, CD26, CD156, and CD249) control the shedding and processing of cytokines and cell adhesion molecules. Given the important regulatory functions of these enzymes, specific inhibitors of their activities might be useful as tools to modulate immune functions [25].

Small molecule inhibitors and antibodies offer two possible strategies for inhibiting leukocyte ecto-enzymes [1, 25]. While small chemical antagonists often lack specificity, antibodies have the capacity to discriminate between closely related enzymes. As leukocyte cell surface ecto-enzymes are accessible to antibodies, the remarkable preference of hcAbs for binding into the active sites of enzymes offers the possibility to raise selective enzyme-blocking sdAb-reagents [5, 6]. The crystal structures of several leukocyte ecto-enzymes have been elucidated, and in many cases they display a deep active site crevice that may be suitable for targeting by sdAbs (Fig. 8a, b).

In a recent proof-of-principle study, we have generated sdAbs against the toxin-related murine T-cell ecto-ADP-ribosyltransferase ART2.2 (Table 1; Fig. 7a) [29]. This cell surface enzyme plays an important regulatory role in inflammatory settings, where it senses NAD released from damaged cells [30‐32]. ART2.2 mediates NAD-induced cell death of naive T-cells by ADP-ribosylating the cytolytic P2X7 receptor [33, 34]. Three out of four distinct ART2.2-specific VHHs isolated from an immune llama phage display library were found to block the enzymatic activity of ART2.2 [29]. Following intravenous injection, a VHH (s+16a) was able to completely block ART2.2 enzymatic activity and NAD-induced cytotoxicity on the surface of T-cells in peripheral lymphatic organs in vivo. This blockade was effective from within 10 min for at least 6 h after injection. Subsequently, we constructed a chimeric single domain antibody composed of VHH s+16a fused to the Fc-domain of mouse IgG1 (Fig. 7f). Following i.v. injection, these reconstituted chimeric hcAbs achieved complete blockade of ART2.2 on lymph node cells within 2 h after injection. This somewhat slower inhibition than sdAbs likely reflects the slower tissue penetration capacity of the larger Fc-fusion protein. The blockade of ART2.2 by s+16a-Fc was effective for >7 days, reflecting the higher in vivo half-life due to slower elimination via the kidney and a high affinity interaction with the anti-catabolic FcRn molecule [35]. These antibodies provide unique tools to specifically block the activity of a single member of the ART-family of ecto-ADP-ribosyltransferases. Our ongoing studies address the potential therapeutic utility of these antibodies in autoimmune and other inflammatory diseases.

By analogy to ART2.2, other leukocyte ecto-enzymes with deep active site crevices, such as CD38 (Fig. 8b), CD39, CD203, and CD151 are potential therapeutic targets [25]. With the goal of generating and cloning enzyme-blocking sdAbs against these enzymes, we have initiated immunizations of llamas and sharks with some of these enzymes.

Leukocytes and other cells express numerous other functionally important membrane proteins that are not enzymes, including receptors, ion channels and transporters. Recently, nanobodies directed against the CD16 Fc-receptor on natural killer (NK) cells were developed from an immunized llama [36]. Once multimerized via biotin/Streptavidin interactions, these sdAbs were capable of activating NK cells, which makes them good candidates to be used in bi-specific formats for recruiting NK cells to target and destroy tumor cells. From a llama immunized with the human A431 squamous cancer cell line, an sdAb specific for the α3β1 integrin (VLA-3) was isolated that could inhibit the binding of K562 erythroleukemia cells to the extracellular matrix [37].

In search of an agent that could be used to surmount the blood brain barrier, a llama was immunized with brain vascular endothelial cells [38]. One of the selected sdAbs, directed against α 2-3 sialoglycoprotein, was found to induce transcytosis of the sdAb through and release on the basolateral side of endothelial cells [39]. In vivo studies showed that this sdAb was efficiently transported across the blood brain barrier and could even be used to carry genetically fused cargo into the brain, including the entire M13 phage particle [38].

Several sdAbs have been raised against receptors over-expressed in tumors and/or tumor neovasculature with the goal of generating tools for improving immune therapy of tumors. Carcinoembryonic antigen (CEA, also named CD66e) is highly expressed on cancer cells of epithelial origin, but not at all or only at low levels by adult cells. A CEA-specific sdAb derived from an immunized dromedary (cAb-CEA5) was fused to a β-lactamase, an enzyme that can convert non-toxic prodrugs into potent cytotoxic agents [40]. The fusion protein was found to localize preferentially in tumors and to be cleared rapidly from the circulation via the kidney. The combined treatment with sdAb-lactamase fusion protein and an anticancer prodrug induced regressions and cures of established tumor xenografts [40]. Several groups have raised sdAbs against the epidermal growth factor receptor (EGFR) [41‐44]. Both, monovalent (17 kD) and bivalent (35 kD) formats showed rapid tissue penetration as well as rapid clearance via the kidney. Conjugation of monovalent sdAb to technetium-99 m (Fig. 7b), a short lived gamma ray emitting isotope widely used in nuclear medicine, permitted in vivo discrimination of EGFR expressing tumors [43]. Tandem fusion of two anti-EGFR VHHs with a VHH specific for serum albumin (Fig. 7e) yielded a 50 kD single-chain reagent that also exhibited excellent tissue penetration [44] with faster and deeper tumor penetration than cetuximab, a conventional chimeric monoclonal antibody currently used in the therapy of metastatic colorectal cancer and squamous cell carcinoma of the head and neck. Fusion to the albumin-specific sdAb resulted in markedly enhanced serum half-life [44].

sdAbs recognizing mucin MUC1, an epithelial cell surface protein over-expressed during lactation and in breast cancer, and endoglin (CD105), an endothelial cell surface protein associated with tumor neovascularization, were isolated from immunized dromedaries and camels [45, 46], yielding potential tumor targeting agents for cancer therapy.

In recent years, cytokine-neutralizing conventional antibodies have revolutionized the therapy of rheumatoid arthritis, psoriasis, and other chronic inflammatory diseases [21, 47]. The great success of these antibodies has prompted the quest for developing smaller, cytokine-neutralizing sdAbs from camelids and sharks. TNF-α blocking sdAbs have been isolated from a llama immunized with human and mouse TNF-α [16] as well as from alpacas immunized with ovine TNF-α [48]. Tandem fusion of two human TNF-α specific VHHs (Fig. 7d) yielded bivalent sdAbs with increased avidity and a more than 100-fold increased TNF-α neutralizing activity [16]. These bivalent sdAbs were markedly more effective at neutralizing TNF-α than the conventional TNF-α blocking antibodies infliximab and adalimumab. Fusion to a third VHH specific for serum albumin (Fig. 7e) yielded potent TNF-α neutralizing sdAbs with enhanced serum half-life and enhanced targeting to inflamed joints in a mouse model of collagen induced arthritis. Daily treatments initiated at the onset of clinical symptoms significantly reduced clinical severity. These sdAbs are currently being developed for clinical trials [49].

Like cytokines, other secreted molecules that are functionally important are accessible to sdAbs. Nanobodies have been developed against components of the blood clotting cascade and against aggregation-prone proteins implicated in amyloid diseases. An sdAb (AU/VWFa-11) derived from an immunized llama specifically recognizes the activated form of von Willebrand factor (vWF), a key component of the blood-clotting cascade that mediates the tethering of platelets to the vascular endothelial wall [50]. This sdAb proved useful for determining the levels of activated vWF in serum samples, although it did not block the interaction between vWF and its cognate integrin GpIbα on the endothelial cells. Another vWF-specific sdAb has recently completed phase Ib clinical trial as an anti-thrombotic reagent, showing dose dependent essentially complete inhibition of vWF for a duration of 4 to 18 h for single dose applications and up to 24 h in multi-dose applications (http://​www.​Ablynx.​com).

Amyloidosis is a group of diseases caused by unusual aggregation of proteins into amyloid fibrils. The extracellular accumulation of amyloid Aβ peptide in insoluble aggregates is associated with Alzheimer’s disease. The use of specific antibodies for inhibiting fibril formation seems a promising therapeutic strategy [51]. An sdAb (cAb-HuL6) derived from a dromedary immunized with wild type human lysozyme [52] inhibited the in vitro aggregation of the amyloidogenic D67H variant of lysozyme [12]. An Aβ-specific VHH (V31-1) isolated from an Aβ-peptide-immunized alpaca, was shown to preferentially bind to the oligomeric form of Aβ. Moreover, this VHH inhibited fibril formation and protected neurons in vitro from Aβ-induced neurotoxicity [53]. A conformation sensitive VHH (B10), selected from a fully synthetic camelid library recognizes an epitope common to different amyloid fibrils derived from Aβ peptide, Ig light chains, and serum amyloid protein [54].

sdAbs directed against human serum albumin and IgG, isolated from immune llama libraries, have been shown to be useful for the bulk depletion of these proteins from serum. Removal of immunoglobulins can have therapeutic benefit, for patients with high titers of circulating autoantibodies, e.g., Goodpasture syndrome and systemic lupus erythematosus (SLE) [55]. sdAb-based affinity-columns showed superior performance to conventional protein A columns. sdAbs directed against human and mouse serum albumin were shown to increase the in vivo half-life of bispecific sdAb reagents (Fig. 7e).

The cell membrane is impermeable to proteins and, therefore, intracellular antigens are inaccessible to secreted antibodies. Two experimental strategies have been pursued to allow access of sdAbs to intracellular targets: the fusion of sdAbs to peptides mediating translocation across the cell membrane and transfection of cells with cDNA constructs encoding intracellular sdAbs. Several natural proteins are equipped with specialized domains that mediate membrane translocation. In some cases, transplantation of such a translocation domain (td) onto another protein has yielded constructs capable of membrane translocation, albeit with highly variable efficiencies. Such td-fusion proteins were constructed of a conventional single-chain variable fragment (scFv) against the Bcl-2 anti-apoptotic protein [56] and against the c-myc oncogene [57]. Very high concentrations of these scFv-td fusion proteins had to be added to cells to yield detectable pro-apoptotic effects on Bcl-2 or anti-proliferative effects on c-myc. The requirements for protein translocation across the cell membrane are poorly understood, but it is likely to involve retrograde transport to an internal compartment, unfolding of the protein, translocation through a channel, and refolding in the cytosol. It is conceivable that the presence of intrachain disulfide bonds could hamper the unfolding and translocation steps and that replacement of the canonical cysteine pair in the Ig-domain (Fig. 2) might improve translocation efficiency of sdAbs.

The alternative strategy involves transfection of cells and co-expression of the sdAb in the same cellular compartment as the target protein. sdAbs appear particularly suited for this strategy as they can readily fold in different environments, including the cytosol, nucleus, and chloroplasts. Cell death induced by oxidative stress is involved in pathologies of post-mitotic tissues, such as brain and heart. Bax, a 24 kD protein of the Bcl-2 family, is involved in the early phase of apoptosis and has been implicated in neuronal death in ischemia. Specific Bax inhibitors hold promise as therapeutic tools for neurodegenerative disorders. Bax-specific sdAbs isolated from a naïve llama library were shown to inhibit Bax induced permeabilization of isolated mitochondria. Cell lines stably transfected with expression constructs for cytosolic anti-Bax sdAbs were found to be resistant to oxidative stress and showed significantly reduced H₂O₂-induced cytotoxicity [18].

Hypoxia triggers a transcriptional response, largely mediated by hypoxia-inducible factor-1alpha (HIF-1α) which regulates the metabolic adaptation to reduced oxygen levels. This metabolic adaptation allows tumor cells to thrive despite a low-blood supply in growing tumors. HIF-1α-specific sdAbs selected from a non-immune llama phage display library were shown to be useful for diagnostic purposes and for immunoprecipitation of HIF-1α. These sdAbs are currently being studied for possible functional modulation of HIF-1α function when expressed as intrabodies [58, 59].

Overexpression and/or trinucleotide repeat expansion in the coding region of poly-A-binding protein 1 (PABN1) can result in fibril-like aggregation of PABN1 in the nuclei of skeletal muscle cells, resulting in oculopharyngeal muscular dystrophy (OPMD). A PABN1-specific sdAb was selected from a non-immune llama library and fused in-frame to a nuclear localization signal and green fluorescent protein [60]. Transfection of this construct into HeLa cells confirmed successful expression and tagging to the nucleus. The construct was found to completely block aggregation of co-expressed PABN1 and was even capable of clearing already existing aggregates when transfected after PABN1. In a Drosophila model of OPMD, expression of a PABN1-specific intrabody strongly suppressed muscle degeneration, leading to nearly complete rescue.

These findings underscore the potential of using sdAbs for targeting intracellular as well as extracellular proteins. A transfection-based expression strategy seems, in principle, applicable for transgenic animals and plants, as well as in therapeutic settings, where cells transfected in vitro are reapplied to the patient. Direct in vivo transfection has been achieved for skin cells using ballistic DNA-immunization and for the liver using hydrodynamic transfection by intravenous bolus injections of DNA in a large volume. In general, however, in vivo transfection efficiencies are low. The unusually long CDR3 of many camelid and shark antibodies could potentially open the possibility of creating synthetic peptide mimetics [61]. In a recent proof-of-principle study, a peptide derived from the CDR3 of the lysozyme blocking camel Ab cabLys3 (Fig. 3b) was shown to block lysozyme activity. However, much higher concentrations of this “peptibody” were required for inhibiting lysozyme than of the parental VHH [62]. Nevertheless, it is possible that synthetic peptibodies can be delivered more efficiently to the cytosol than larger sdAbs.

The higher immunogenicity of repetitive proteins could potentially be exploited also for increasing vaccine efficiency. Lumazine synthase from Brucella abortus spontaneously assembles into pentamers and decamers and shows very high immunogenicity even during primary immunizations [63]. In our own labs, we have raised sdAbs specific for lumazine synthase as tools to interfere with flavin biosynthesis in these pathogenic bacteria. These sdAbs might also be useful for converting a monomeric antigen into a multimeric format by providing a “piggyback” link to lumazine synthase, e.g., following cloning and recombinant production of an sdAb-antigen fusion protein (Fig. 7i). We hypothesize that the antigen’s immunogenicity will be much higher in such a molecular assembly vaccine than in its original monomeric format.

Antibodies directed against the antigen-binding paratope (idiotype) of another antibody are called anti-idiotypic antibodies (anti-Id Abs) (Fig. 7j). Such anti-Id Abs compete with the antigen for binding to the parental Ab and are thought to share partially or completely the reactive surface with the combining site, thus mimicking the structure of the antigen. In a proof-of-principle study, two sdAbs were selected from a llama immunized with a DNA-specific monoclonal antibody (ED84) [64]. These sdAbs were found to compete with a double-stranded DNA oligonucleotide for binding to ED84 and thus bear a functional mimicry of the DNA. In a similar approach anti-idiotypic VNARs were selected from a semi-synthetic phage display library against the idiotype of a monoclonal antibody specific for a linear tripeptide epitope in the Plasmodium falciparum apical membrane protein antigen-1 (AMA-1). These VNARs showed enhanced affinity compared to the parental antigen [65].

Some of the most potent toxins found in nature are enzymes, including numerous proteins secreted by bacteria and the venomous organs of snakes and scorpions. Toxin-specific sdAbs have been isolated from immune llama phage display libraries (Table 2). Like murine ART2.2, the SpvB Salmonella toxin is an ADP-ribosyltransferase that transfers ADP-ribose from NAD onto specific amino acid side chains of proteins [26, 31] (Fig. 8c). Upon translocation into the cytosol of animal cells, SpvB blocks cytoskeletal functions by ADP-ribosylating actin and thereby blocking its polymerization. From an SpvB-immunized llama, we have isolated SpvB-blocking sdAbs that block SpvB enzymatic activity at a 1:1 molar ratio. When expressed as intrabodies in transfected cells, these nanobodies effectively block SpvB-mediated disruption of the cell cytoskeleton (Fig. 7h). Several other important human pathogens, including V. cholera, B. pertussis, and S. enterica, which induce disease by secreting toxin ARTs [68] might similarly be suitable for targeting by enzyme-blocking sdAbs.

Bacterial toxin-specific sdAbs have been successfully isolated from naïve llama and naïve shark libraries, including sdAbs specific for cholera toxin, staphylococcal enterotoxin B, and botulinum neurotoxin A [69‐71]. It is likely that many of these sdAbs possess only low affinities and their neutralization capacities have not been reported. Notwithstanding, some have been shown useful for diagnostic purposes, e.g., upon conversion to a multivalent format by the attachment to beads. These sdAbs, thus, hold promise for biosensor applications.

Endotoxin, i.e., lipopolysaccharide (LPS), released from the outer membrane of gram-negative bacteria into the blood circulation after infection can mediate severe septic shock. LPS-specific sdAbs, selected from a non-immune llama VHH phage display library, effectively removed LPS from serum and other aqueous solutions [72].

Venom of snakes, scorpions, spiders, and frogs represent significant threats to human health in many countries. Intravenous administration of conventional IgG or Fab fragments prepared from immunized horses or sheep can provide alleviation for some cases of systemic envenoming and recent studies indicate that corresponding hcAbs prepared from immunized camels and llamas are similarly or even more effective [73‐75]. An sdAb against alpha cobra toxin was derived from a naïve llama phage display library and engineered into a pentameric format by genetic fusion to the non-toxic B-subunit of verotoxin, yielding a reagent with higher avidity and neutralization capacity [76]. An sdAb directed against scorpion toxin (NbAahI′22) derived from an immunized dromedary showed potent neutralizing capacity already in a monovalent format which was enhanced further upon conversion into a bivalent heavy-chain format by genetic fusion to the Fc region of human IgG1 (Fig. 7f) [77].

Fungi secrete small chemical compounds that can be toxic to animal cells. Recently, an sdAb (NAT-267) was developed from an immunized llama against BSA-conjugated Deoxynivalenol (300 Da), a mycotoxin produced by common grain molds [78]. The high affinity of this sdAb paves way for developing low-cost toxin detection systems and potentially also for immunomodulation of Deoxynivalenol’s cytotoxic effects. Other haptens have been targeted by sdAbs, including red azo dyes (728 Da) found in red wine (with the potential use of targeting enzymes to stains during washing) [79], caffeine (with the potential use of detecting caffeine in beverages) [80], the explosive TNT (biosensor) [81], and the chemotherapeutic agent methotrexate (MTX, 454 Da) [82]. Anti-hapten sdAbs have potential in cancer therapy, for e.g., in a bi-specific format to help target radiolabeled haptens to tumors for imaging and radioimmunotherapy [83].

Viruses often use “hidden” epitopes on the capsid or envelope, e.g., deep invaginations or canyons, for docking onto receptors on host cells. Such epitopes are hidden from the large and typically flat antigen-binding sites of conventional antibodies. Moreover, these epitopes are often well conserved between viruses that use the same receptor while more protruded regions of the capsid/envelope change rapidly, helping the virus to escape from immune attack. Analogous to the active site crevice of enzymes, these canyons on the viral surface are usually inaccessible to conventional antibodies, but constitute preferential targets for single-chain antibodies from camelids and sharks. Numerous viruses are being targeted by sdAbs, among these HIV, for which crosstype-specific HIV-neutralizing sdAbs were recently derived from an immunized llama [84].

Several groups have succeeded in raising sdAbs directed against rotavirus, the causative agent of severe childhood diarrhea. Immunization of a llama with a more internally located capsid protein of rotavirus yielded sdAbs that recognize and neutralize several different strains of rotavirus [85]. In a mouse model, oral application of these sdAbs provided significant amelioration of diarrhea. In an elegant approach to increase the local dose of rotavirus-absorbing sdAbs, a rotavirus-specific sdAb derived from an immunized camel was cloned in-frame with a lactobacillus cell surface protease into a lactobacillus expression vector [17]. Transformation yielded recombinant versions of this commensal gut bacterium that express a high density of rotavirus-specific sdAbs on the cell surface (dubbed lactobodies) (Fig. 7g). Feeding of the recombinant lactobacillus also provided significant protection from rotavirus-mediated disease.

Foot and mouth disease virus (FMDV) neutralizing sdAbs were isolated from a llama immunized with a crude extract of FMDV-infected BHK cells [86]. The in vivo half-life of these sdAbs was enhanced by genetic fusion to a second VHH specific for pig IgG. Intramuscular administration of these bispecific sdAbs significantly reduced viremia in subsequently infected piglets, but did not prevent transmission of FMDV [87]. Virus-neutralizing sdAbs have also been developed for industrial applications, e.g., for the neutralization of phages that infect lactobacilli and thereby present a significant threat in the milk and cheese industry [88‐90].

sdAbs have been generated and applied successfully also against intracellular viral proteins. Porcine endogenous retroviruses (PERV) reside within the pig genome and are capable of transfecting human cells in vitro, thereby presenting a potential hazard for xenotransplantation. An sdAb, isolated from a llama immunized with the 60 kD Gag protein of porcine endogenous retrovirus [91] and expressed as an intrabody, effectively inhibited production of virus particles. Transgenic expression of such sdAbs could contribute to solving safety problems associated with PERVs. Similarly, an sdAb derived from a llama immunized with hepatitis B virus particles was converted into a format targeted to the lumen of the endoplasmic reticulum (ER) by fusion to the KDEL ER-retention signal. Cells co-transfected with expression constructs for this sdAb reagent and hepatitis B showed enhanced intracellular retention and correspondingly diminished secretion of hepatitis B particles. In mouse models, the rapid intravenous bolus injection of an expression vector for the hepatitis-specific sdAb in a large volume of buffer resulted in efficient hydrodynamic transfection of liver cells and a therapeutic benefit [20].

Four sdAbs were selected from a non-immune llama library by panning against immobilized Marburg virus [92]. Even though these sdAbs from a naïve library displayed only low affinities, a sensitive diagnostic assay system could be developed by converting the nucleoprotein-specific sdAbs into multivalent formats. Capture of the virus was achieved by simple immobilization of the sdAbs on the wells of an ELISA plate. Detection was achieved upon genetic fusion to bacterial alkaline phosphatase which forms homo-dimers, thus yielding a bivalent enzyme-linked detection reagent. The assay allows detection of three Marburg virus variants without cross reactivity to the four Ebola virus species.

Single domain antibodies have also been raised against bacterial surface proteins with the goals of blocking attachment of bacteria to host cells and/or for more effective delivery of pro-drugs. A VHH (K609) against E. coli F4 fimbriae, applied at high doses, reduced E. coli induced diarrhea in piglets. However, when applied orally, a large portion of this sdAb was found to be proteolytically degraded in the upper gastrointestinal tract. Proteolytic degradation is mediated by pepsin in the stomach and by trypsin and chymotrypsin in the duodenum [93]. Using a DNA-shuffling strategy involving the random recombination of gene segments obtained from VHH K609 and two other F4-specific VHHs, following DNA fragmentation and PCR amplification, a variant VHH (K922) was selected that exhibited >100-fold increase in resistance to in vitro degradation by gastric and jejunal fluid.

Streptococcus mutans, the main cause of dental caries, achieves virulence in part via its ability to adhere to and to accumulate on tooth surfaces. An sdAb (S36-VHH) was developed from a llama immunized with a mixture of disrupted and intact S. mutans and genetically fused to fungal glucose oxidase, a homo-dimeric 150 kD enzyme that produces hydrogen peroxide (H₂O₂) in the presence of glucose and oxygen. The fusion protein was readily produced in yeast [94]. In a rat model of strong cariogenic challenge, daily application of the sdAb monomer as well as the VHH-glucose oxidase fusion protein resulted in a reduced level of bacterial recovery 21 days after infection, albeit not at a statistically significant level due to large variation in bacterial recovery [95].

sdAbs have also been used successfully to improve the probability of obtaining crystals for proteins that are difficult to crystallize. An sdAb can assist the crystallization process by stabilizing the protein of interest and/or by creating additional crystal contact surface. VHHs isolated from immunized llamas were used to assist the crystallization of major components of the type 2 secretion system of enteropathogenic E. coli and Vibrio. This 11 component extracellular protein secretion system is used for secretion of cholera toxin and heat labile enterotoxin [96, 97]. Similarly, the phage P2 receptor-binding protein was successfully co-crystallized with an sdAb [98].

Many bacteria secrete enzymes that inactivate antibiotics as an efficient escape mechanism from antibiotic therapy. Enzyme-blocking sdAbs were generated from immunized dromedaries that effectively block β-lactamases which catalyze the hydrolysis of β-lactam antibiotics [99] with the potential utility of improving antibiotic therapies.

Single domain antibodies are also being developed as anti-parasite reagents. Trypanosomes are coated with variable surface glycoproteins (VSGs) and evade host immune responses by changing the VSGs that cover the entire membrane. Owing to their smaller size, sdAbs might better penetrate the VSG coat to gain access to conserved epitopes that are inaccessible for conventional antibodies. An sdAb (NbAn33) recognizing an oligomannose cryptic epitope on trypanosomes was isolated from a dromedary immunized with purified VSG [100]. In contrast to conventional antibodies and lectins, this sdAb was able to penetrate the dense VSG coat. A fluorochrome-conjugate of this sdAb allowed the direct sensitive detection of parasites in blood smears. Immunotoxins were generated by genetically fusing this sdAb (An33) to either β-lactamase or a truncated version of the pore forming human serum protein apolipoprotein L-I (Fig. 7c). Fusion to β-lactamase, which can convert a cephalosporin-based prodrug into the potent toxic phenylenediamine mustard (PDM), yielded a conjugate with both, retained capacity to bind to trypanosomes and retained enzyme activity. Sub-lethal doses of the pro-drug proved highly toxic to conjugate treated parasites. Genetic fusion to a truncated form of human apolipoprotein L-I yielded an immunotoxin with lytic activity against a range of different trypanosomes [101]. In a mouse model the VHH-apoL-I fusion protein had curative and alleviating effects on acute and chronic infections with trypanosomes. Further sdAbs directed against VSG and other trypanosome proteins were selected from an immune library obtained from a dromedary infected with live trypanosomes [102]. We have also obtained and characterized sdAbs directed against trans-sialidase from Trypanosoma cruzi [103]. Even though these sdAbs are unable to interfere with the crucial role of trans-sialidase in cell entry of the parasite, they served as a tool to unmask a novel strategy of T. cruzi to evade the host immune system.

The larval form of the pork tapeworm Taenia solium is the cause of cysticercosis, the most common parasitic infection of the central nervous system and a major cause of epilepsy in developing countries. Immunization of two dromedaries with T. solium extracts yielded an sdAb (Nbsol52) recognizing the 14 kD diagnostic glycoprotein Ts14 [104]. This sdAb showed subnanomolar affinity for Ts14 and was found capable of capturing antigen present in the serum of infected pigs. Further studies are required to assess the potential therapeutic use of such sdAbs in cysticercosis. Malassezia furfur is a fungus implicated in dandruff. sdAbs specific for a cell wall protein of M. furfur were derived from an immunized llama [105]. Some of these sdAbs were found to be resistant to detergents in shampoo and may provide a possible antifungal additive to shampoos.