The increasing load of environmental pollutants poses a serious threat over the globe. In this vulnerable situation, it is essential to have alternative sources of medicines, may be from invertebrates. Among invertebrates, although molluscs are known for their consumption as food and ethno‐medicinal use, the importance of these animals is still overlooked. Presently attention has been geared toward molluscs including Achatina fulica which are now considered as one of the most evolutionary successful animals. During the last few decades, researchers are trying to decipher their complex immune system to harvest valuable molecules to treat human diseases. In the present review, the existence of important immunological factors in Achatina is discussed addressing the coagulation system, innate immune molecules, bioactive proteins and lastly the enigmatic C‐reactive proteins.
- Achatina fulica
- innate immunity
- antibacterial activity
Extensive research on invertebrate immune system for the last few decades, including molluscs, revealed that invertebrates contain peptides which are endowed with anti‐microbial activity . These peptides can trigger specific anti‐bacterial reaction by producing different isoforms specific for each bacterial species. Among immunological molecules of invertebrates, Toll‐like receptor 4 (TLR4) gained much attention, though its essentiality happens to be more pronounced in vertebrates . Gastropod diversity is well documented, recording 40,000–150,000 species with size variance of 1 mm to 1 m and indicating a strong immune system in gastropods [1–3].
The giant African snail,
Terrestrial snails are well known for accumulating heavy metals in their tissues and serve as a pertinent species for monitoring trace metals, agrochemicals, urban pollution and electromagnetic exposures . The effect of accumulated heavy metals in different molluscan tissues and possible use of such alterations as biomarkers of exposure to xenobiotics has been investigated in some detail [8, 9]. Although snails are considered as alleged pest they are used by humans for various purposes including vigorous consumption of mollusc meat in several countries around the globe, including tribal and urban populations of India and Bangladesh . Another important aspect is the ethno‐medicinal use of several mollusc species highlighted by several authors [11, 12]. Pharmacological application of different body parts of mollusc are used to treat several diseases which suggests its potential to act as a source of drug . In the present chapter, various characters of
2. Molecules in the Innate Immune System of
2.1. Coagulation system in
Invertebrates are not able to synthesize immunoglobulins, rather they have developed a potential defense system against microbial surface antigens such as lipopolysaccharides (LPS)/endotoxins and glucans . Among various kinds of innate immune mechanisms in invertebrates, two types of coagulation mechanisms are on record: (i) in crustaceans such as lobster, crayfish  and insects  clotting occurs through Ca‐dependent transglutaminase, (ii) serine protease zymogens dependent coagulation system is reported which is similar to mammalian system . In
3. Acharan sulfate, the new glycosaminoglycan from
Acharan sulfate, a glycosaminoglycan isolated from
4. Anti‐bacterial protein from mucus of
Achacin is an antibacterial glycoprotein obtained from the mucus present on the body surface of
5. Role of Snail Hemocytes in Innate Immunity
Circulating blood cells known as hemocytes represent the main cellular component of the molluscan immune system. Hemocytes are composed of a mixture of different subpopulations of cells, for example, flow cytometric analyses of hemocytes from the freshwater snails
If attention is focused on the functional attributes of hemocytes, several reports in this direction revealed diverse immunological functions such as phagocytosis , cytotoxicity , aggregation  and pathogen encapsulation [37, 38]. In addition to hemocytes, hemolymph, the humoral component of the molluscan immune system, is reported to exhibit the activities of superoxide dismutase , catalase  and acid  and alkaline phosphatases . Total hemocyte count in mollusc has been considered as an important immune parameter . Elevation of the total hemocyte count indicates augmentation of immunity of invertebrates . Phagocytosis is an established strategy of immune defense in invertebrates including mollusc. It is considered as the major immunological activity evidenced in many molluscan species . Major cytotoxic molecules such as superoxide anion and nitric oxide generated by the circulatory hemocytes of molluscs are functionally associated with the destruction of pathogens [46, 47]. Phenoloxidase is reported to be functionally associated with phagocytosis, self‐nonself discrimination, cytotoxicity and melanization response . Superoxide dismutase and catalase play a significant antioxidation role in the cellular physiology of molluscs. In addition, glutathione‐S‐transferase is functionally associated with general detoxification response of xenobiotics and anti oxidation activity . All these enzymes are involved in scavenging and deactivating the toxic oxidative radicals and protect the tissue from oxidative damage . Acid and alkaline phosphatases are functionally involved in pathogen destruction in phagolysosome which bear immunological significance . Several reports also demonstrated a range of receptors which bind carbohydrates, extracellular matrix proteins, hormones, growth factors and cytokines resulting specific immunocyte signals not only in vertebrates but also in molluscs . Thus, it can be surmised that signaling systems are evolutionary conserved functions of immunocytes in the animal kingdom.
Apart from the above‐mentioned defense mechanisms, snails also undergo starvation and aestivation under any stress condition. Though several reports are available on starvation and aestivation of snails, information on immune‐related parameter of Indian mollusc is scant. In
5.1. C‐reactive protein (CRP), a multifunctional player in Achatina
C‐reactive protein (CRP) was first discovered in Oswald Avery’s laboratory at the Rockefeller Institute for Medical Research . CRP has evolved conservatively, and homologous proteins with similar functional attributes have been found in many other species. The stable preservation of this protein during evolution implies some biological significance. Thus, CRP is an ancient molecule discovered in humans only about 82 years ago. It belongs to a protein family called pentraxin (from the Greek words “penta” five and “ragos,” berries) that constitutes a phylogenetically ancient family of proteins exhibiting a remarkable conservation of structure and binding reactivities. The presence of CRP has been reported from a wide range of different animals such as monkey, dog, goat, rabbit, rat, mice, domestic fowl, fish, shark and lumpsucker among vertebrates and horseshoe crab  and
Several authors reported that CRP can protect mice from infections caused by both Gram‐positive
It is also noted that bacterial cells are strongly dependent on metabolic cycles for their survival and pathogenicity [71, 72]. Therefore, effect of
Several authors  reported potentiality of human CRP to inhibit superoxide (O2−) generation and delay apoptosis in neutrophils . Recently, it has been reported that immunepotent CRP modulates antioxidant and anti‐inflammatory effects in LPS‐stimulated human macrophages . The anti‐stress property of ACRP was tested in mice which are known to have a very low level of endogenous CRP (∼2 μg/mL) even after an inflammatory stimulus . In order to prove this hypothesis, lead nitrate was administered intraperitoneally at an environmentally relevant dose in mice, and the induced oxidative stress was found to be removed when ACRP was administered prior to treating with Pb
In molluscs, several anti‐microbial peptide (AMP) genes are triggered during onset of a broad range of pathogenic infections. Furthermore, several categories of immune molecules are extracted from snails including glycosaminoglycans, peptides, proteins (glycoproteins) and enzymes which possess diverse biological activities [78, 79]. Interestingly, evolutionary success of
It was earlier established that xenobiotics, like heavy metals, are successful in triggering the synthesis of CRP causing inflammatory condition, and in turn, CRP was found to be a very good scavenger in eliminating these heavy metals. In contrast to human and other higher level mammals, the normal fresh water teleost
Snails accumulate heavy metals more in their tissue inducing numerous acute and sublethal effects . Due to this sensitivity, they are considered as excellent bioindicators of heavy metal contamination . The effect of accumulated heavy metals on different molluscan tissues and possible use of such alterations as biomarkers of exposure to xenobiotics has been investigated [8, 9]. Molluscs have shown considerable promise as biomonitors of metal pollution , and an extensive literature has appeared concerning mechanisms of uptake, detoxification and storage of heavy metals . Few studies on several fresh water and marine species further substantiate the role of molluscs as bioaccumulators . Further, ecological and ecophysiological studies suggest that molluscs react to environmental stress and pollution by modifying their behavior . It is reported that terrestrial snails might regulate some metals assimilated from food and xenobiotic exposure . The kinetics of metal accumulation and detoxification are still a subject of discussion, and there is a lack of information regarding metal toxicity in snails [84, 87].
Presently immunological molecules in mollusc, especially