CCR5 and CCR5Δ32 in bacterial and parasitic infections: Thinking chemokine receptors outside the HIV box

The CCR5 molecule was reported in 1996 as the main HIV‐1 co‐receptor. In that same year, the CCR5Δ32 genetic variant was described as a strong protective factor against HIV‐1 infection. These findings led to extensive research regarding the CCR5, culminating in critical scientific advances, such as the development of CCR5 inhibitors for the treatment of HIV infection. Recently, the research landscape surrounding CCR5 has begun to change. Different research groups have realized that, since CCR5 has such important effects in the chemokine system, it could also affect other different physiological systems. Therefore, the effect of reduced CCR5 expression due to the presence of the CCR5Δ32 variant began to be further studied. Several studies have investigated the role of CCR5 and the impacts of CCR5Δ32 on autoimmune and inflammatory diseases, various types of cancer, and viral diseases. However, the role of CCR5 in diseases caused by bacteria and parasites is still poorly understood. Therefore, the aim of this article is to review the role of CCR5 and the effects of CCR5Δ32 on bacterial (brucellosis, osteomyelitis, pneumonia, tuberculosis and infection by Chlamydia trachomatis) and parasitic infections (toxoplasmosis, leishmaniasis, Chagas disease and schistosomiasis). Basic information about each of these infections was also addressed. The neglected role of CCR5 in fungal disease and emerging studies regarding the action of CCR5 on regulatory T cells are briefly covered in this review. Considering the “renaissance of CCR5 research,” this article is useful for updating researchers who develop studies involving CCR5 and CCR5Δ32 in different infectious diseases.

disease progression is due to the phenotypic effects of this polymorphism. CCR5Δ32 homozygous individuals do not express CCR5 on the cell surface, while heterozygous individuals have reduced CCR5 expression (Brelot & Chakrabarti, 2018;Venkatesan et al., 2002;Wu et al., 1997). These findings drew much attention from the scientific community looking for genetic resistance factors against HIV infection. Since then, a high number of studies addressing the relationship between CCR5 and HIV have emerged. A PubMed search on February 2020 using the terms "HIV" and "CCR5" resulted in over 6,000 published articles.
The role of CCR5 in conditions unrelated to HIV infection has been neglected for many years. However, recently the research involving the influence of CCR5 on different conditions has been intensified. Such studies address the CCR5 molecule per se (Castanheira et al., 2019;Jiao et al., 2018;Liu et al., 2018;Umansky, Blattner, Gebhardt, & Utikal, 2017), CCR5 pharmacological blockade (Moy et al., 2017;Pervaiz et al., 2019), CCR5 gene editing (Xie, Zhan, Ge, & Tang, 2019) and the genetic variant CCR5Δ32 (Fatima et al., 2019;Kaminski, Ellwanger, Sandrim, Pontillo, & Chies, 2019;Kletenkov et al., 2019;Słomiński et al., 2017;Toson et al., 2017;Troncoso et al., 2018). The resurgence of research involving these different aspects of CCR5 is due to two main factors: the advancement of TA B L E 1 CCR5 human haplogroups gene-editing technologies, which allow the exploration of the metabolic and pathophysiological effects of CCR5 editing (Allen et al., 2018;Qi et al., 2018;Vangelista & Vento, 2018;Xu et al., 2017), and the development of CCR5 blockers for clinical use (Vangelista & Vento, 2018). Recently, the different effects of CCR5 editing were much debated Wang & Yang, 2019;Xie et al., 2019) after a Chinese scientist alleged to had performed this procedure on human embryos (Cyranoski & Ledford, 2018). Such a claim generated a series of criticisms regarding the ethical and clinical issues involved in this procedure (Krimsky, 2019;Wang, Li, Li, Gao, & Wei, 2018;Wang & Yang, 2019). Although the number of studies involving CCR5 over the years has remained relatively stable, interventions such as these have increased attention regarding studies with CCR5.
CCR5Δ32 has complex and variable effects, which may be "deleterious" or "protective" according to the disease or condition evaluated Lim, Glass, McDermott, & Murphy, 2006). The absence of CCR5 due to CCR5Δ32 may not be optimally compensated for by other chemokine receptors, inducing undesirable effects on the immune system in the face of specific immune challenges . These features make the investigation of the roles of CCR5Δ32 on different diseases even more interesting.
Therefore, the lack of CCR5 is deleterious in some situations, but blocking this receptor may be desirable in specific clinical contexts.
If we understand how the CCR5Δ32 variant is expressed and where and how the truncated molecule act, it will be possible to infer the effects of CCR5 absence or low expression on different diseases without (or prior to) the need for gene-editing technologies or CCR5 pharmacological blockade. Also, gene-disease association studies are important tools for the discovery of potential therapeutic targets (Hill, 2012;Segal & Hill, 2003), especially considering the existence of CCR5 blockers already approved for use in humans (Vangelista & Vento, 2018). Thus, to investigate the effects of CCR5Δ32 (and potentially of the low expression of CCR5) in distinct physiological and pathological conditions will help to define where and when the use of CCR5 blockers could be advantageous. Additionally, this type of study allows the discovery of genetic factors of susceptibility or resistance to different infectious diseases (Frodsham & Hill, 2004;Hill, 2012;Segal & Hill, 2003). In this sense, the main objective of this article is to review the role of CCR5 and the impacts of CCR5Δ32 in different bacterial and parasitic infections, which are little-explored fields in the context of CCR5 research. The following bacterial infections are covered in this article: brucellosis, osteomyelitis, pneumonia (Mycoplasma pneumoniae and Streptococcus pneumoniae), Tuberculosis and infection by Chlamydia trachomatis. Regarding parasitic infections, toxoplasmosis, leishmaniasis, Chagas disease and schistosomiasis are included in this review. Considering the variety of infections addressed, basic information about each of them is also described. Also, the structure of the CCR5 gene and protein, some relevant CCR5 polymorphisms, and evolutionary aspects of CCR5Δ32 are addressed. Finally, the role of CCR5 in fungal diseases and emerging studies regarding the combined action of CCR5 and regulatory T cells are briefly discussed.

| The structure of the CCR5 gene and its polymorphisms
The CCR5 gene is located on chromosome 3 (cytogenetic band 3p.21.31), has a length of 6,065 bases and is composed of three exons and two introns. CCR5 ORF (Open Reading Frame) contains 1,056 bases and is located on exon 3. The protein generated by its translation has a total of 352 residues (Hoover, 2018;Liu et al., 1996;Mummidi et al., 2000). Spatially, the CCR5 is very close to the CCR2, another chemokine receptor gene, the last one being upstream to CCR5 (Lawhorn et al., 2013). Figure 1 (panel b) schematizes CCR5 gene structure and flanking regions. Also, there is a recently described long noncoding RNA (lncRNA) gene overlapping with CCR5, termed CCR5AS, whose expression is positively correlated with CCR5 mRNA levels (Kulkarni et al., 2019). This finding is quite relevant due to the impact of noncoding genetic elements on the susceptibility and progression of infectious diseases (Ellwanger, Zambra, Guimarães, & Chies, 2018).

| Origin and evolutionary aspects of CCR5Δ32
CCR5Δ32 is a genetic variant originating from a single mutation event that occurred in Europe Libert et al., 1998;Stephens et al., 1998). Due to its European origin, the Δ32 allele frequency is higher in Euro-descendant populations. The highest allele frequencies are observed specifically in populations in northern Europe, in countries such as Denmark, Estonia, Finland, Latvia, Lithuania and Norway. In these specific countries, the allele frequency of the CCR5Δ32 is higher than 12% (Solloch et al., 2017).
Migratory flows and miscegenation processes have spread the Δ32 allele among different human populations. For these reasons, the CCR5Δ32 can be found at relatively high frequencies in populations outside the European continent, as in Brazil, where it is not difficult to observe the presence of the variant in the southern region of the country Silva-Carvalho et al., 2016).
It was estimated that the origin of the CCR5Δ32 in the human population occurred about 700 years ago (considering a range of 275-1,875 years) (Stephens et al., 1998). An origin between 1,400 and 3,500 years ago was estimated in another work (Libert et al., 1998). However, studies evaluating ancient DNA showed that the CCR5Δ32 might be older than these estimates, being prevalent in prehistoric Europeans (Bouwman, Shved, Akgül, Rühli, & Warinner, 2017;Faure & Royer-Carenzi, 2008;Hedrick & Verrelli, 2006;Hummel, Schmidt, Kremeyer, Herrmann, & Oppermann, 2005;Zawicki & Witas, 2008). Sabeti et al. (2005) estimated that the origin of the CCR5Δ32 in the human population occurred more than 5,000 years ago, which is in agreement with evidence pointing to the presence of the Δ32 allele in samples of ancient DNA. Of note, there is additional evidence pointing to the presence of the Δ32 allele for even 7,000 years ago (Faure & Royer-Carenzi, 2008).
The origin area of the CCR5Δ32 may not be the same as the current areas of the highest Δ32 allele frequency . Considering its spread, Vikings may have contributed to the dissemination of the Δ32 allele across Europe Lucotte, 2002;Lucotte & Dieterlen, 2003;Novembre et al., 2005). Negative selection during the Roman expansion may also have played a role in determining the currently observed frequency of the Δ32 allele. Interestingly, Romans could have contributed to the decrease of allele frequency in ancient European populations in which the allele was highly prevalent (Faure & Royer-Carenzi, 2008).
Climatic conditions and geographical characteristics may also have affected, directly or indirectly, the distribution and frequency of the Δ32 allele (Balanovsky et al., 2005;Limborska et al., 2002).
Selective pressures can have acted on CCR5Δ32, favouring its expansion in the human population (Libert et al., 1998;Stephens et al., 1998). Although CCR5Δ32 homozygous genotype protects against HIV (an adaptive advantage considering a scenario of high HIV circulation), this pathogen was not responsible for increasing the frequency of the variant in the human population, since the HIV/AIDS pandemic is recent in human history (Galvani & Slatkin, 2003). Different authors have tried to discover the potential selective pressures responsible for the fixation of the CCR5Δ32 in the genome (>1% rate) and subsequent expansion of the Δ32 allele in the European population. It was hypothesized that an epidemic in Europe could have been an essential selective event that acted on the Δ32 allele (Carrington et al., 1997;Stephens et al., 1998). In this case, CCR5Δ32 would have been a protective factor against a particular infectious disease, giving an evolutionary advantage to individuals bearing the Δ32 allele. Consequently, the CCR5Δ32 increased in frequency and spread in the European population. In other words, the allele would have undergone positive selection.
The Black Death occurred between 1,346 and 1,352, representing the historically most important epidemic of bubonic plague, an infectious disease caused by the bacterium Yersinia pestis. During Black Death, 25%-33% of Europeans died from bubonic plague.
Black Death and other bubonic plague outbreaks were hypothesized as a central selective pressure on CCR5Δ32, since Y. pestis infection cases occurred in epidemic proportions in the geographical region where the CCR5Δ32 is currently observed in high frequency. The period of occurrence of Black Death (~650 years ago), together with the first estimates that pointed to a recent origin of the CCR5Δ32 (~700 years ago), helped to support this hypothesis (Stephens et al., 1998). The connection between bubonic plague, CCR5Δ32 and HIV has become widely known in the scientific community and among the lay public through the media (Stumpf & Wilkinson-Herbots, 2004). However, various studies based on mathematical models, historical evidence, population data, animal models of Y. pestis infection and ancient DNA analysis did not support this hypothesis (Baron & Schembri-Wismayer, 2011;Bouwman et al., 2017;Cohn & Weaver, 2006;Galvani & Slatkin, 2003;Hummel et al., 2005;Mecsas et al., 2004;Styer, Click, Hopkins, Frothingham, & Aballay, 2007). Of note, CCR5-/-mice are not resistant to Y. pestis infection (Mecsas et al., 2004;Styer et al., 2007). For some authors, Black Death was caused by a viral haemorrhagic fever instead of bubonic plague. This viral disease would have been fundamental to increase the frequency of CCR5Δ32 in Europe, representing an alternative hypothesis .
In large populations, most new genetic mutations are lost over generations (Stephens et al., 1998). It is essential to consider that the occurrence of a particular mutation in the genome is different from fixing it in the population at a frequency higher than 1%, at which point the mutation is called "genetic variant" or "polymorphism." If the frequency of CCR5Δ32 has increased in the European population due to infectious diseases outbreaks, it is possible that different diseases or epidemics may have had a cumulative/synergistic effect on the allele and the selective pressures responsible for its maintenance in the population should not necessarily be attributed to a particular epidemic or pathogen. Moreover, the CCR5Δ32 can be advantageous or harmful, depending on the situation and disease considered Vargas et al., 2009). Therefore, one cannot exclude the possibility that additional selective pressures nonrelated to infectious diseases have acted on the Δ32 allele.
In the 1990s, it was already hypothesized that the frequency of CCR5Δ32 could have increased in the European population through genetic drift (as a neutral mutation). However, at that time, this process was considered unlikely (Libert et al., 1998;Stephens et al., 1998). Sabeti et al. (2005) carried out a robust study regarding the evolutionary aspects of CCR5Δ32 and concluded that neutral evolution is the main process responsible for the observed frequencies of the Δ32 allele in humans. However, the authors did not rule out the possibility that some selective pressure may have acted on CCR5Δ32 in a remote historical period (Sabeti et al., 2005).
Brucella spp. is commonly transmitted by the ingestion of contaminated/unpasteurized milk of sheep, cow, goat and camel. In addition, other forms of transmission have also been reported, such as congenital, sexual and breastfeeding (Harrison & Posada, 2018).
Brucella spp. infection can also occur via inhalation, damaged skin and conjunctiva (Franco et al., 2007). Cases of paediatric infection are common, mainly in endemic countries. In low-income countries, paediatric infection is associated with contact of children with animals and ingestion of unpasteurized milk (Alshaalan et al., 2014;Bukhari, 2018).
Pronounced inflammation is a key feature of Brucella-infected tissues. One interesting characteristic of brucellosis is the apparent milder inflammatory response in the affected organism, but the longterm presence of the pathogen leads to tissue damage through the production of cytokine and chemokines (Baldi & Giambartolomei, 2013;Krishnan, Kaplin, Graber, Darman, & Kerr, 2005;Seidel, Pardo, Newman-Toker, Olivi, & Eberhart, 2003). Considering the potential role of CCR5 in the immune response against Brucella infection and interactions of the pathogen with CCR5 + cells, Skendros, Boura, Tsantas, Debre, and Theodorou (2002) evaluated the influence of CCR5∆32 in both susceptibility and outcome of human brucellosis. Of note, it was observed a deviation from the Hardy-Weinberg equilibrium within the brucellosis group due to the proportion of CCR5∆32 homozygous patients (2 out of 185 individuals). Although the authors have suggested that this result may indicate an association of CCR5∆32 with the disease, no statistically significant difference in the CCR5∆32 allele frequency between patients with brucellosis and controls was detected. Also, no difference was found between patients with acute brucellosis and those with chronic/ relapsing brucellosis (Skendros et al., 2002). In general terms, the available data do not support a pivotal influence of CCR5∆32 on Brucella spp. infection. However, to the best of our knowledge, only the study of Skendros et al. (2002) evaluated the CCR5∆32 in the context of brucellosis so far.

| Chlamydia trachomatis infection
Chlamydia trachomatis infection is registered worldwide, with an overall prevalence of 1%-6% (Rawre, Juyal, & Dhawan, 2017). It was estimated that approximately 90 million people are infected by C. trachomatis each year (Starnbach & Roan, 2008). This infection is sexually transmitted and is a common cause of nongonococcal urethritis (Rawre et al., 2017). Infection by C. trachomatis is generally an asymptomatic and nonfatal disease, but untreated infection can lead to chronic production of pro-inflammatory cytokines, resulting in tissue damage and inflammation-related diseases, including pelvic inflammatory disease. In the long term, this condition in women can cause ectopic pregnancy and tubal factor infertility.
Other consequences of long-term infection are the development of epididymitis and proctitis in men (Brunham & Rey-Ladino, 2005;Rawre et al., 2017;Starnbach & Roan, 2008). Moreover, one of the possible complications after infection is the emergence of postvenereal reactive arthritis, a form of inflammatory arthritis (Carter & Hudson, 2009).
The chemokines CXCL9, CXCL10 and the CCR5 ligand CCL5 are upregulated in cells from the upper genital tract after C. trachomatis infection (Maxion & Kelly, 2002). Also, two studies performed with mice have shown that CCR5 has a fundamental role in T cell-mediated clearance of C. trachomatis in genital mucosa (Barr et al., 2005;Olive, Gondek, & Starnbach, 2011). In humans, the presence of CCR5Δ32 was associated with increased C. trachomatis burden, based on the measurement of bacterial chromosome copies in synovial biopsies (Gérard et al., 2010). Taking into consideration that CCR5Δ32 may affect the activity of inflammatory cells involved in C. trachomatis-associated diseases, the potential impacts of this polymorphism on C. trachomatis infection was assessed in a few studies. Barr et al. (2005) evaluated the effect of CCR5∆32 on the risk of developing tubal pathology in Caucasian subfertile women seropositive for C. trachomatis infection. Although no statistically significant differences in the CCR5∆32 frequency between subfertile women and controls were found, a higher frequency of CCR5∆32 was observed in women seropositive for C. trachomatis without tubal pathology as compared to women with tubal pathology (Barr et al., 2005). This result suggests that, in the context of C. trachomatis infection, the CCR5∆32 could protect against tubal pathology.
However, the sample size used in this specific analysis was limited (n = 41 in total), and this result must be considered with prudence.
Moreover, the CCR5∆32 was not a protective factor of tubal pathol- Finally, Carter et al. (2013) investigated the effect of CCR5∆32 on the susceptibility to C. trachomatis-associated reactive arthritis in a sample of the North American population, but no association between the polymorphism and the disease was found. Taking together, the results mentioned above do not support a critical role of CCR5Δ32 in the development of C. trachomatis-associated diseases.

| Osteomyelitis
Osteomyelitis is a bone inflammation associated to infections (mainly Staphylococcus aureus). Healthy bones have high resistance to infections. Thus, osteomyelitis is more likely to occur among patients presenting conditions such as decubitus ulcers, surgery, traumas, intravenous drug use and diabetes (Chihara & Segreti, 2010). Diabetic foot ulcers and diabetic foot infections often trigger osteomyelitis (Malhotra, Chan, & Nather, 2014). The disease causes destruction of the bone and can occur in different clinical forms: secondary to a contiguous focus of infection, secondary to vascular insufficiency, or from haematogenous origin. Osteomyelitis may develop acutely or chronically (Lew & Waldvogel, 2004). Furthermore, osteomyelitis can be classified into 12 different clinical stages, combining the anatomical type affected by the disease (medullary, superficial, localized or diffuse) with the patient's physiological state (Birt, Anderson, Bruce Toby, & Wang, 2017). The disease encompasses a wide range of symptoms: chills, fever, fatigue, irritability, pain, local swelling, lethargy and malaise (Chihara & Segreti, 2010;Lew & Waldvogel, 2004). Due to physiological and anatomical characteristics of bones, osteomyelitis is a difficult-to-treat condition (Lew & Waldvogel, 2004).
It was suggested that CCR5 is important for S. aureus pathogenesis De Souza et al., 2015).
Specifically,  have shown that CCR5 acts as a receptor for S. aureus leukotoxin ED (LukED), a toxin that promotes cell death, and that CCR5-deficient mice are strongly protected from lethal S. aureus infection.
De Souza et al. (2015) performed a cohort study in a sample of the Northeast Brazilian population, aiming to assess the influence of the CCR5∆32 polymorphism on the risk of developing osteomyelitis after bone traumas. As expected, S. aureus was the main pathogen responsible for osteomyelitis in the studied patients. Interestingly, most of ∆32 allele carriers were found in the group of patients who did not develop osteomyelitis. This result suggests a protective effect of the CCR5∆32 against osteomyelitis, but no statistically significant results were found (De Souza et al., 2015).
Although the influence of CCR5 and CCR5Δ32 on osteomyelitis has been sparsely studied, when considered together, the results obtained in mice  and humans (De Souza et al., 2015) indicate that CCR5 and the CCR5Δ32 may play essential roles on the development of S. aureus-associated osteomyelitis. These topics should be studied more intensely, mainly because the use of

| Mycoplasma pneumoniae infection
Mycoplasmas are the smallest self-replicating bacteria with the capacity of cell-free existence. Some mycoplasmas are causative agents of human diseases. Among them, Mycoplasma pneumoniae is responsible for 20%-40% of community-acquired bacterial pneumonia cases during epidemics (Waites & Talkington, 2004;Waites, Xiao, Liu, Balish, & Atkinson, 2017), although other authors mention variable percentages (Parrott, Kinjo, & Fujita, 2016). China, Russia, Mexico and Brazil are among the countries with the highest number of M. pneumoniae cases per 100,000 people (Parrott et al., 2016).
Mycoplasma pneumoniae is a human-specific obligate parasite that inhabits epithelial tissues, such as those of the urogenital and respiratory tracts (Waites & Talkington, 2004;Waites et al., 2017).
When the pathogen reaches the respiratory tract, it invades epithelial cells, causing cilia destruction and compromising the respiratory capacity. Such events are accompanied by an important inflammatory reaction in the lungs (Waites & Talkington, 2004). M. pneumoniae pneumonia occurs in adults and children but is generally more harmful to the second group (Parrott et al., 2016). The severity of the infection ranges from mild to life-threatening. Of note, M. pneumoniae can cause asthma and persistent cough, and even promote extrapulmonary manifestations located mainly in brain, skin, but also other organs (Parrott et al., 2016;Waites et al., 2017).
A set of evidence points to a role of CCR5 in the pathophysiology of distinct respiratory diseases (Bracke, Demedts, Joos, & Brusselle, 2007;Capelli, Stefano, Gnemmi, & Donner, 2005;Dawson, Beck, Kuziel, Henderson, & Maeda, 2000). Host immune factors affect the response against M. pneumoniae infection and modify the course of lung disease (Waites et al., 2017). In this context, Ungvári et al. and chronic M. pneumoniae infection. Also, they found that M. pneumoniae-infected children with the ∆32 allele have a reduced risk of developing asthma as compared to ∆32 allele noncarriers (Ungvári et al., 2007). These findings, as well the data presented in some of the next topics here discussed, indicate that, although the role of CCR5 in respiratory infections is still a neglected topic, it should be explored in greater detail.

| Streptococcus pneumoniae infection
Streptococcus pneumoniae can cause respiratory diseases in humans, including community-acquired pneumonia. Infection by this bacterium affects mainly children, elderly people, and individuals with a deficient immune function. In some cases, the disease can be fatal (Brooks & Mias, 2018). As expected, different cell receptors, chemokines and host cell pathways are involved in the immune responses against S. pneumoniae. The inflammatory profile of each individual can significantly influence such responses, affecting bacterial pathogenesis and transmission (Brooks & Mias, 2018). In this context, there is evidence that CCR5 and its ligand CCL5 have some influence on the interactions between S. pneumoniae and the host (Palaniappan et al., 2006).
S. pneumoniae (alone or together with other infectious agents) was detected as the causative agent of most cases of pneumonia in the evaluated patients. In their study using multiple SNP analysis, the wild-type genotype of CCR5 was a risk factor for community-acquired pneumonia. Therefore, the ∆32 allele is a potentially protective factor against the disease (Salnikova et al., 2013). However, it is essential to consider that this is a population-specific finding. This study should be replicated in other populations so that broader conclusions are possible.

| Tuberculosis
Tuberculosis is an infection caused by bacterial species belonging to the Mycobacterium tuberculosis complex. The infection affects mainly the lungs but may also occur in other organs (Pai et al., 2016).
Of note, individuals exposed to Mycobacterium tuberculosis can eliminate the pathogen without clinical manifestations. If elimination does not occur, the individual may develop latent or active infection.
The disease will manifest depending on the immunological conditions and comorbidities of the host (Pai et al., 2016). Tuberculosis is the leading cause of death among human infectious diseases, being considered one of the main public health concerns nowadays (Azad, Sadee, & Schlesinger, 2012;Furin, Cox, & Pai, 2019). Drug-resistant tuberculosis is a growing problem, making the epidemiological situation of this disease even more worrying. It is estimated that each year more than 10 million new cases of tuberculosis occur worldwide (Furin et al., 2019).
Historically, humans and M. tuberculosis have a long and strong host-parasite interaction. This interaction left marks on the genome of both organisms. As a consequence, the susceptibility to tuberculosis varies in different human populations (Azad et al., 2012).   (Mamtani et al., 2011;Mummidi et al., 2000). The haplotype CCR5-HHD (in which CCR5∆32 wild-type allele is present; see Table 1 for complete haplotype description) was associated with increased CCR5 expression and was considered a risk factor for tuberculosis (Mamtani et al., 2011). Therefore, in accordance with studies mentioned above (Das et al., 2014;Juffermans et al., 2001), increased CCR5 expression may contribute to greater susceptibility to tuberculosis, as well as to disease pathogenesis (Mamtani et al., 2011). Unfortunately, studies evaluating the effect of CCR5∆32 as an individual factor in the context of tuberculosis were not available. Thus, although haplotypic analyses (Mamtani et al., 2011) suggest that the wild-type genotype of CCR5 is a risk factor for disease development, this approach needs to be explored in further studies. Transmission of T. gondii by organ transplantation or during pregnancy may also occur (Montoya & Liesenfeld, 2004). Around 30% of the world's population has serological evidence of T. gondii infection, a particularly important problem in HIV-infected individuals . In most human cases (~90%), the primary infection is asymptomatic. However, some infected individuals can develop cervical lymphadenopathy, heart problems or ocular manifestations (chorioretinitis). Latent infection is generally asymptomatic, but reactivation can occur in immunocompromised patients, leading to toxoplasmic encephalitis. Infection during pregnancy is associated with birth defects, including blindness, microcephaly and intracranial calcifications (Montoya & Liesenfeld, 2004).

Toxoplasmosis is a parasitic infection caused by
Host cell lysis, hypersensitivity and inflammatory responses triggered by T. gondii infection are the main responsible factors for the health problems observed in immunocompetent and immunocompromised patients. Therefore, cytokines and chemokines are essential regulators of toxoplasmosis immunopathology (Gaddi & Yap, 2007). In this context, several studies have shown that CCR5 signalling has different and fundamental roles in T. gondii infection.
Human genetic traits, especially variants in HLA, TLR and cytokine genes, have a fundamental involvement in the development of ocular toxoplasmosis (Fernández, Jaimes, Ortiz, & Ramírez, 2016).
Polymorphisms of the CCR5 gene could impact ocular toxoplasmosis (De Faria Junior et al., 2018). Also, there is a body of evidence supporting an involvement of the CCR5∆32 variant in toxoplasmosis. Meyer et al. (1997) evaluated the effect of CCR5∆32 on HIV disease progression in a sample of Caucasian individuals. Looking at results regarding CCR5∆32 and toxoplasmosis, this infection was observed only in wild-type individuals, suggesting that the heterozygote genotype could have a protective effect on T. gondii infection (Meyer et al., 1997). The protective effect of the ∆32 allele on toxoplasmosis development in HIV-infected individuals was confirmed in later studies (Ashton et al., 2002;Meyer et al., 1999). However, these results should be interpreted with caution, as the scenario of treatment and prevention of HIV infection has changed immensely since they were conducted.

| Leishmaniasis
Leishmaniasis is a tropical vector-borne infection caused by parasites from Leishmania genus. The transmission of leishmaniasis between mammalian species occurs through the bite of sandflies (Burza, Croft, & Boelaert, 2018;Pace, 2014 (Burza et al., 2018). After malaria, leishmaniasis is the deadliest parasitic disease in humans, with a mortality rate of 10%-20% in low-income countries (Pace, 2014).
Leishmaniasis is endemic in approximately 100 countries (Alvar et al., 2012;Burza et al., 2018). However, few countries (Bangladesh, Brazil, India, Ethiopia, Kenya, Somalia, South Sudan and Sudan) encompass more than 90% of worldwide cases of visceral leishmaniasis, and therefore, the disease is considered an important public health issue in these countries (Alvar et al., 2012;Burza et al., 2018;Pace, 2014). Noteworthy, the rise of international travelling has been facilitated the number of new leishmaniasis cases in nonendemic countries (Pace, 2014).
Leishmania spp. infection can be asymptomatic or cause cutaneous, mucocutaneous, or visceral disease (Burza et al., 2018;Pace, 2014). The clinical outcome of this disease depends on the infecting species, vector biology and the host immune response (Burza et al., 2018). The levels of CCR5 + cells and CCR5 expression are increased in HIV/Leishmania co-infected individuals (Nigro et al., 2007;Vallejo et al., 2015). Interestingly, CCR5 facilitates parasite persistence by modulating the migration of regulatory T cells in the host in an animal model of L. major infection (Yurchenko et al., 2006). In a preliminary analysis, these findings suggest that CCR5∆32 could reduce this phenomenon and even promote some protection against leishmaniasis. However, studies with humans do not point in this direction.
Brajão de Oliveira et al. (2007) assessed the influence of CCR5∆32 in the progression from cutaneous to mucocutaneous leishmaniasis in a Brazilian population sample (from Paraná State). The allele frequencies were not statistically different between leishmaniasis patients and healthy controls. Regarding disease progression/severity, the ∆32 allele was related to a protective effect on these aspects, but also without statistical significance. The small sample number used in this study (100 individuals in each group) probably influenced these results (Brajão de Oliveira et al., 2007). Subsequently, another study evaluating the effect of CCR5∆32 on the cutaneous leishmaniasis pathogenesis was performed with individuals from Paraná State, Brazil (Ribas et al., 2013). This study found no statistically significant difference when genotypes were compared between leishmaniasis patients and controls. On the other hand, the CCR5∆32 heterozygous genotype was associated with disease recurrence, suggesting that the ∆32 allele may be a risk factor for the pathogenesis of the disease (Ribas et al., 2013). However, this result should be interpreted with caution due to the very small sample size employed in this specific analysis.
Finally, Sophie et al. (2016) analysed the frequency of the CCR5∆32 in Pakistani individuals with cutaneous leishmaniasis.
However, CCR5∆32 was not associated with protection or susceptibility to leishmaniasis when patients were compared to controls.
Taken together, the three studies reviewed here (Brajão de Oliveira et al., 2007;Ribas et al., 2013;Sophie et al., 2016) suggest that CCR5∆32 has no major effect on leishmaniasis development. Looking at host genetics, HLA alleles and variants in cytokine genes impact both resistance and susceptibility to Chagas diseases and the associated health problems (Ayo et al., 2013;Vasconcelos et al., 2012).

| Chagas disease
Moreover, a body of evidence supports the involvement of the CCR5 protein (Batista et al., 2018;Dutra, Rocha, & Teixeira, 2005;Hardison et al., 2006;Kroll-Palhares et al., 2008;Machado et al., 2005;Marino et al., 2005;Medeiros et al., 2009;Roffe et al., 2019;Roffê et al., 2010;Silva et al., 2007), as well as gene variants of CCR5 and CCR5 ligands (Batista et al., 2018;Calzada et al., 2001;Flórez et al., 2012;Machuca et al., 2014;Oliveira et al., 2015Oliveira et al., , 2016 on varied aspects of Chagas disease, mainly associated to the development of Chagas heart disease. For example, animal-based evidence pointed to a protective role of CCR5 in controlling T. cruzi replication and maintaining a protective immune response in acute infection (Hardison et al., 2006). On the other hand, CCR5 may participate in the inflammation and heart damage observed in Chagas heart disease (Batista et al., 2018;Roffe et al., 2019), suggesting that CCR5 deficiency might play a protective role against Chagas-associated cardiomyopathy. Therefore, CCR5 has two basic conflicting effects on Chagas disease: it is protective in the acute phase and acts as a facilitator of disease-related inflammation in chronic infection . Interestingly, the potential use of pharmacological CCR5 modulators to treat Chagas disease has already been suggested by different authors (Machado et al., 2005;Marino et al., 2005;Medeiros et al., 2009;Roffe et al., 2019). In the opposite direction, Silva et al. (2007) demonstrated that CCR5 has no relevant role in T. cruzi infection-related meningoencephalitis. However, meningoencephalitis is a rare manifestation of this infection (Silva et al., 2007). Moreover, CCR5 alone did not have a relevant role in mucosal protection against T. cruzi, although the CCL5-CCR5 axis is important in such protection (Sullivan et al., 2011). Calzada et al. (2001) attempted to evaluate the effect of CCR5Δ32 on Chagas disease in a Peruvian population sample. However, no individual evaluated had a homozygous genotype, and the number of heterozygous was very low (three individuals between cases and controls; Δ32 allele frequency: 0.009). For these reasons, the authors could not evaluate the effect of CCR5Δ32 on Chagas disease (Calzada et al., 2001). The low CCR5Δ32 frequency in a Peruvian population sample is not a surprising result, as it is known that the allele is uncommon in Native American populations (Calzada et al., 2001;Vargas et al., 2006). Similarly, in the study by Flórez et al. (2012) with individuals from Colombia, it was not possible to evaluate the effect of CCR5Δ32 on Chagas heart disease due to the low frequency of the Δ32 allele observed in patients. However, CCR2/ CCR5 haplotype analysis was performed in this same study (considering nine polymorphisms: one of the CCR2 gene and eight of the CCR5 gene). The human haplogroup A was statistically associated with a higher risk for Chagas disease-associated cardiomyopathy.
The human haplogroup A includes the CCR5Δ32 wild-type allele, but the specific haplotype promotes low transcriptional activity of CCR5 gene, which can impair the CCR5-related control of T. cruzi infection and promote the cardiac problems associated with the infection (Flórez et al., 2012).
Finally, the potential effect of CCR5Δ32 on Chagas disease-associated heart problems was assessed in populations from Venezuela (Fernández-Mestre, Montagnani, & Layrisse, 2004) and Brazil (Oliveira et al., 2014(Oliveira et al., , 2015. However, no statistically significant results were found in these studies. Considering the data discussed above, it is clear that CCR5 has different and relevant impacts on Chagas disease, such as protection against the parasite and mediating inflammation in chronic disease, but the genetic variant CCR5Δ32 seems to have little effect on such CCR5-related impacts.  (Gazzinelli et al., 2017;Mbanefo et al., 2014).

| Schistosomiasis
An exacerbated inflammatory response against Schistosoma eggs may contribute to schistosomiasis pathogenesis (McManus et al., 2018). According to Souza, Sousa-Pereira, Teixeira, Lambertucci, and Teixeira (2006), the chemokines CCL3 and CCL5 along with the chemokine receptors CCR1 and CCR5 modulate the clinical course of S. mansoni infection. Thus, chemokines and chemokine receptors are key modulators of the granulomatous inflammation process during Schistosoma infection (Souza et al., 2011;Souza, Souza, Negrão-Correa, Teixeira, & Teixeira, 2008). Also, by combining pieces of evidence, it was hypothesized that a predominant CCL5/CCR5associated immune response is linked to the mild form of schistosomiasis whereas a predominant CCL3/CCR1-associated immune response would be linked to more severe forms of the disease (Souza et al., 2006). Other chemokine receptors, such as CCR2 and CCR4, apparently act similarly to CCR1 (Souza et al., 2008). In brief, these receptors may be important drivers for granuloma formation and fibrosis development in response to antigens from Schistosoma eggs.
On the other hand, the CCL5/CCR5 pathway decreases disease severity by controlling inflammation, fibrosis and collagen deposition in addition to the recruitment of T regulatory cells (FoxP3+ cells) to the granulomatous lesions (Souza et al., 2008(Souza et al., , 2011. Therefore, the classical link "lack of CCR5 -less inflammation" seems not to be true in schistosomiasis. The role of CCR5 in this infection is regulatory (Souza et al., 2008(Souza et al., , 2011. As will be briefly discussed in the section "Role of CCR5 on T regulatory cells," the regulatory action of CCR5 is still a little-explored field, but it may answer important questions related to CCR5 functions. Animal and in vitro studies suggest an involvement of CCR5 in different aspects of schistosomiasis pathogenesis (Liang et al., 2012;Richardson et al., 2014;Souza et al., 2011). CCR5-deficient mice show more severe Schistosoma infection and mortality rate than CCR5 wild-type mice, suggesting a protective role of CCR5 against aggressive granuloma formation during the course of infection (Souza et al., 2011) , 2014;Secor et al., 2003;Silveira-Lemos, Teixeira-Carvalho, Martins-Filho, Oliveira, & Corrêa-Oliveira, 2006 (Kleppa et al., 2014;Secor et al., 2003).

| Summarizing what we know
Details of the studies investigating the impacts of the genetic variant CCR5Δ32 on bacterial and parasitic infections are summarized in Tables 2 and 3

| CCR5 in fungal diseases
Studies are showing the involvement of CCR5 in some fungal diseases, including Aspergillus fumigatus-induced asthma in mice (Schuh, Blease, Brühl, Mack, & Hogaboam, 2003;Schuh, Blease, & Hogaboam, 2002), and infections by Candida albicans (Kim et al., 2005), Paracoccidioides brasiliensis (Moreira et al., 2008), Histoplasma capsulatum in mice (Kroetz & Deepe, 2010, 2012 and Coccidioides (Davini et al., 2018). Due to the limited number of studies that have addressed CCR5 in the context of human fungal infections, it is not yet possible to establish the real importance of this receptor in the clinical course of these diseases. However, taking into account the studies carried out so far, together with the known impacts of CCR5 on infections caused by other pathogens, we speculate that CCR5 could play important but little-explored roles in fungal diseases.
The potential effect of CCR5 blockers on fungal infections is another overlooked topic. It is unlikely that pharmacological CCR5 blockage affects susceptibility to fungal infections. However, this is a topic that should be further investigated, particularly in immunosuppressed patients (Merchant, Reichman, & Koval, 2007).

| Role of CCR5 on regulatory T cells
Pieces of evidence point to a role of CCR5 in the mediation of recruitment and action of regulatory T (Treg) cells in fungal (Davini et al., 2018;Kroetz & Deepe, 2010), bacterial (Ahmed et al., 2018), parasitic (Romano et al., 2016;Souza et al., 2011;Yurchenko et al., 2006) and viral (Kim et al., 2016) infections. CCR5 + Treg cells have immunosuppressive activity (Soler et al., 2013) and CCR5 signalling mediates the recruitment of Treg cells to inflammation sites, thus modulating inflammatory responses by inducing a Treg-mediated immunosuppressive environment (Dobaczewski, Xia, Bujak, Gonzalez-Quesada, & Frangogiannis, 2010;Doodes et al., 2009;Li et al., 2017;Velasco-Velázquez, Xolalpa, & Pestell, 2014;Wildenberg et al., 2008). Therefore, lack or reduced expression of CCR5 due to CCR5∆32 may contribute to exacerbated inflammation and be associated with a worse prognosis in infection-related inflammatory states ( Figure 3). For example, in schistosomiasis, the recruitment of Tregs cells via CCR5 towards granulomatous lesions has important impacts on disease outcomes (Souza et al., 2008(Souza et al., , 2011. These aspects are quite interesting and can shed light on the mechanisms involved in the pathogenesis of different diseases. Moreover, the F I G U R E 2 Impacts of CCR5Δ32 on different infectious diseases. "Reduced expression" is a phenotype observed in CCR5Δ32 heterozygotes. In CCR5Δ32 homozygotes, the CCR5 expression on the cell surface is absent. For more details, see the topic "4.1. Summarizing what we know"

F I G U R E 3 CCR5 regulatory action on infection-related inflammation.
Inflammation induced by Schistosoma spp. eggs are illustrated in the figure. The CCR5 molecule mediates the migration of regulatory T cells to inflammation sites, thus modulating inflammation. Reduced or absent CCR5 expression (due to CCR5Δ32 heterozygosity and homozygosity, respectively) can impair the migration of regulatory T cells to inflammation sites. This figure was created using Servier Medical Art illustrations (available at https://smart.servi er.com, under a Creative Commons Attribution 3.0 Unported License) regulatory action resulting from the CCR5-Treg cells interaction may be a promising therapeutic target. However, the regulatory action of CCR5 is a still neglected topic that also needs further investigation.
Future studies may help explain the multiple and often contradictory effects of CCR5 on different infections.

| CON CLUS IONS
Based on the infections discussed in this review, it is clear that the influence of CCR5 and CCR5∆32 in different diseases should not be generalized. The role of these two factors is quite variable in different conditions and may also vary in distinct clinical times of the same disease. For example, both CCR5 and CCR5Δ32 impact the development of osteomyelitis, although in Chagas disease, CCR5 is involved in the disease pathogenesis, and the CCR5Δ32 variant has little influence. In schistosomiasis, on the other hand, the action of CCR5 seems to be based on regulatory mechanisms. Also due to the different roles in distinct pathological conditions, the use of CCR5 blockers would be beneficial in some clinical situations, such as HIV infection. However, their effects on the inflammatory response in the context of other infections can be quite variable and even harmful. Therefore, the clinical use of CCR5 blockers must be investigated in detail in each clinical scenario rather than being regarded as a general rule.

CO N FLI C T S O F I NTE R E S T
The authors declare no conflicts of interest.