SLPI belongs to the whey acidic protein four-disulfide core family of proteins. The human SLPI gene is localized on chromosome 20q12-13.2 (Kikuchi et al. 1998). The SLPI gene consists of four exons and three introns, it spans approximately 2.6 kb (Kikuchi et al. 1998; Stetler et al. 1986). The SLPI gene is stable and seems to be nonpolymorphic (Abe et al. 1991). Though, it has the potential to be modulated at both the transcriptional and post-transcriptional levels (Abe et al. 1991). Up to date, it has not been detected a state of SLPI deficiency. However, patients with severe congenital neutropenia (a primary immunodeficiency syndrome characterized by mutations in at least 6 different genes) were found to have strongly reduced SLPI levels, being SLPI a key factor for the neutrophil differentiation in the bone marrow (Klimenkova et al. 2014).

The SLPI gene is actively transcribed in mucosal cells, being the half-life of the transcripts of approximately 12 h. Close to the exon 1, SLPI gene has four potential binding sites for transcription factor AP-1, three for AP-2 and one for C/EPB(Klimenkova et al. 2014). Also, Kikuchi et al., describes that SLPI has a promoter region which has a recognition sequence for two transcription factor, one of which is highly expressed in lung cell lines, and the other in nonlung cell lines (Kikuchi et al. 1997).

SLPI is an 11,7 kDa molecular weight non-glycosylated protein composed by 132 amino acids (Stolk et al. 1999). The amino acid sequence of SLPI generates a highly polycationic peptide with two highly homologous domains. These two domains (COOH and NH2 terminal domains) share around a 35% homology (Vogelmeier et al. 1996). Each domain contains eight cysteine residues that form four disulfide bonds, which helps to stabilize the structure of the molecule (Grutter et al. 1988). These cysteine rich domains are also called WAP domains (Whey Acid Protein). Domain 2 was initially described to bind and inhibit the serine proteases such as trypsin and elastase, while the domain 1 was probably not inhibitory (Eisenberg et al. 1990; Meckelein et al. 1990). It has been proposed that this last domain helps in the stabilization of the complexes "SLPI:elastase". Also, it is believed that the domain 1 mediates binding to heparin, and thus increases its antiprotease activity, probably as a result of a conformational change of the molecule. (Faller et al. 1992).

SLPI was first isolated from bronchial secretions (Hochstrasser et al. 1972; Ohlsson et al. 1976). Then the SLPI was characterized by two groups of researchers, whom purified the molecule from the urine and (Seemuller et al. 1986) and the parotid gland secretions (Thompson et al. 1986). SLPI is located in both, the extracellular matrix and the intracellular compartments, suggesting that it could exert autocrine and paracrine effects (Taggart et al. 2005).
The expression of SLPI is constitutive as well as modulated by different factors. Constitutively SLPI can be found in serum and in extravascular mucosal fluids. Thus, it is found around of 40 (26.1-65.0) ng/ml in serum, 72 (0.4-250) ng/ml in bronchial lavage fluid (Hollander et al. 2007), in exhaled breath condensate (2.82 - 0.58 pg/ml)(Tateosian et al. 2012) and saliva (0.3-3.2 ug/ml) (Shugars et al. 2001). However, concentrations of the molecule vary depending on age and gender of the individual tested. In vivo, it is produced in the lung by tracheal serous glands and by clear bronchial cells. In male (Ohlsson et al. 1995) and female (Moriyama et al. 1999) genital tracts, SLPI is located in seminal plasma and cervical mucosa, respectively. Furthermore, it is produced by the parotid glands, intestinal epithelial cells (Si-Tahar et al. 2000), renal tubule cells (Ohlsson et al. 2001), keratinocytes (Wiedow et al. 1998), beta cells of the pancreas (Nystrom et al. 1999) and immune cells like neutrophils and alveolar macropaghes (Sallenave et al. 1997; Mihaila et al. 2001; Guerrieri et al. 2011).
The SLPI expression is modulated by different molecules. It has been shown that SLPI is up-regulated by LPS, IL-1beta, TNF-alpha, neutrophil elastasa, alpha-defensins, surfactant protein A, corticosteroid and progesterone (Sallenave et al. 1994; Reid et al. 1999; Maruyama et al. 1994; Abbinante-Nissen et al. 1995; King et al. 2003; Velarde et al. 2005; van Wetering et al. 2000; Ramadas et al. 2009). Finally, apoptotic cells can upregulate SLPI production by macrophages (Odaka et al. 2003). In contrast, few factors can downmodulate the expression of SLPI. Among them, the most significant are IFNgamma and TGF-beta (Jaumann et al. 2000; Jin et al. 1997).
Although, the structure of SLPI seems to be stable, it could be cleaved and inactive-ated by chymase(Belkowski et al. 2008), cathepsins B, L, S (Taggart et al. 2001), lipid peroxidation products (Tomova et al. 1994) and Host dust mite 1 allergen (Brown et al. 2003), among others(Weldon et al. 2009).

Antiprotease activity: The inhibition of protease activity was described for C-terminus domain against elastasa, cathepsin G, trypsin, chymotrypsin, tryptase and chymase (Williams et al. 2006). Thus, SLPI major function is inhibit inflammation by blocking the proteolytic activity of serine proteinases released by leukocytes and also through blocking the LPS effects, such as the upregulation of several cytokines like TNFalpha, MCP-1 and IL-6 (Yang et al. 2005; Jin et al. 1998; Taggart et al. 2005; Ashcroft et al. 2000). SLPI acts locally to maintain a protease/antiprotease balance thereby preventing protease mediated tissue destruction (Vogelmeier et al. 1990). In the lungs, the disturbance of this balance is responsible for various lung diseases, many of which are initiated and maintained by the recruitment and activation of neutrophils (Birrer et al. 1994; Suter 1989).
Anti-inflammatory activity:
SLPI has anti-inflammatory activities not necessarily related to its ability to inhibit extracellular proteases. The anti-inflammatory activity is also mediated by inhibition of proteolytic degradation of IkB, an inhibitor of the nuclear factor NF-kB (Ashcroft et al. 2000; Samsom et al. 2007). It has been shown that over-expression of SLPI inhibits NF-kB, which is a transcription factor of several pro-inflammatory mediators in pulmonary inflammation (Henriksen et al. 2004). Currently, there are some evidence that SLPI is rapidly taken up by cells and is localized in the nucleus and cytoplasm (Taggart et al. 2002). In the cytoplasm, SLPI prevents degradation of several key proteins in the regulated activation of NF-kB, as IkBalpha, IkBbeta and IRAK (IL-1-receptor-associated kinase) through the ubiquitin-proteasome mechanism (Greene et al. 2004; Taggart et al. 2002), that follows the activation of NF-kB by LPS or LTA (lipoteichoic acids). Also it has been proposed that SLPI acting in the nucleus can bind to NF-kB consensus region of target genes (Taggart et al. 2005). The entering into the nucleus occurs through a mechanism in which SLPI may traverse membranes, due to its cationic nature (favored by the high content of arginine and lysine) by interaction with the negatively charged membrane. Independently of the mode of action, in vivo experiments have demonstrated anti-inflammatory / pro-apoptotic activities in the lung, and in a variety of other organs.
Microbicidal activity:
Against Bacteria:
SLPI displays anti-microbial properties in vivo and in vitro (Sallenave 2002; Gomez et al. 2009). It has been recently reported that mouse and even human SLPI shows anti-bacterial activity against mycobacteria and it constitutes a pattern recognition receptor (PRR), that not only kills the microorganism, but also facilitates their phagocytosis by murine and human macrophages (Nishimura et al. 2008; Gomez et al. 2009). Either the antimicrobial activity or PRR ability depends on the COOH terminal domain where the inhibitory activity of serine proteases resides. The WAPs domains of the molecule are involved, and this is due to cationic residues that allow the disruption of the membranes of target organisms (Verma et al. 2007; Gomez et al. 2009; Nishimura et al. 2008). The antimicrobial activity of human SLPI has been described for various bacteria such as Pseudomonas aeruginosa, Staphylococcus aureus, Staphylococcus epidermis (Wiedow et al. 1998; Wingens et al. 1998), Mycobacterium tuberculosis (Gomez et al. 2009), and Escherichia coli (Williams et al. 2006). Therefore this activity is against Gram negative and Gram positive bacteria and is part of the defense system of the mucosa.
Against Viruses:
SLPI has been suggested as the main soluble factor responsible for the HIV inhibitory effect of saliva. It is well-established that human saliva inhibits HIV infectivity in vitro (McNeely et al. 1995; Nagashunmugam et al. 1997; Shugars et al. 2001; Malamud et al. 1992; Fultz 1986). The infection of adherent primary monocytes with HIV-1 was significantly suppressed in the presence of human saliva [76-80]. Four in vitro studies have demonstrated that SLPI has anti-HIV-1 activity in cells that included peripheral blood mononuclear cells, purified primary T cells, and SupT1 cells, a lymphocyte-derived tumor cell line (Fultz 1986; Hocini et al. 2000; Shugars et al. 1997; Skott et al. 2002). Evidence suggests that SLPI blocks HIV-1 internalization in a dose-dependent manner (McNeely et al. 1997). McNeely et al. found that SLPI inhibits a step of viral infection that occurs after virus binding but before reverse transcription. In a co-precipitation experiment, it was described a 55-kDa cell surface protein from monocytes by using anti-SLPI antibodies. For some authors, the interaction between HIV and CCR5 could be the main target of SLPI (Naif et al. 1998). Other authors showed that SLPI interferes with HIV fusion with the T-cell plasma membrane through binding to scramblase 1, a membrane protein that interacts with CD4 and controls the movement of the phospholipid bilayer of the plasma membrane (Shugars et al. 1999). It was also demonstrated that in myeloid cell, SLPI blocks viral entry/fusion as a result of binding to annexin II (Ohlsson et al. 2001; Ma et al. 2004; Drannik et al. 2011). This molecule is a macrophage receptor that binds to phosphatidylserine moiety that HIV carries on its outer layer on exiting from an infected cell (Ohlsson et al. 2001; Drannik et al. 2011; Ma et al. 2004). Furthermore, the elastase inhibiting activity of SLPI was not be essential for their anti-HIV-1 activity (McNeely et al. 1997).
Against Fungi:
C. albicans and Aspergillus fumigatus were sensitive to the antimicrobial activity of recombinant SLPI. This activity was localized to N-terminal domain of the molecule (Tomee et al. 1997).
Wound healing activity:
The role of SLPI in tissue repair was suggested by the observation that in human, epithelial expression of SLPI is increased in damaged skin (Wingens et al. 1998). Studies in SLPI deficient mice demonstrated that SLPI has an essential role in wound healing (Ashcroft et al. 2000). In the absence of SLPI, the animals presents a delay in cutaneous wound healing, which is attributed to an increased and prolonged inflammatory response during the repair process, and a delay in the accumulation of the matrix. The altered inflammatory profile involves enhanced activation of local TGF-beta (Ashcroft et al. 2000).
Immunomodulatory activity in adaptive immune response:
The effect of SLPI seems not to be limited to innate immune response but also to the cellular and humoral adaptive immune response. In fact, the high SLPI expression was found in dendritic cells of mucosal lymph node and it was suggested that these dendritic cells regulate cellular activation to microbial products and maintain the tolerance threshold (Samsom et al. 2007). Also, we have observed that SLPI decreases lymphocyte proliferation, a phenomenon which depends on the presence of monocytes (Guerrieri et al. 2011). However, it is not possible to rule out a direct effect of SLPI on lymphocytes since it is able to bind the receptors phospholipid scramblases 1 and 4 on CD4 T cells (Py et al. 2009). On tonsillar cells, SLPI inhibits B cells expressing activation-induced cytidine deaminase, an enzyme involved in class switching. Thus, the overall idea is that SLPI is a tolerigenic factor, that it is able to down modulate the innate and adaptive immune response. Moreover, recently it has been shown that the hyporesponsiveness of human buccal epithelium to microbial stimulation is a phenomenon that depends on SLPI expression. (Menckeberg et al. 2015).
Recently, it has been also described that SLPI, in conjunction of neutrophil DNA or cathepsin G and human neutrophil elastase, induced a marked production of type I interferon by plasmacytoid dendritic cells (Skrzeczynska-Moncznik et al. 2012; Skrzeczynska-Moncznik et al. 2013). On the other hand, it was found that SLPI inhibits the formation of neutrophil extracellular traps; structures that are involved in the elimination of microorganisms, and also in the presentation of autoantigens (Zabieglo et al. 2015). These findings suggest a role of SLPI in autoimmune diseases.