Etiology of chronic prostatitis/chronic pelvic pain syndrome – How animal models guide understanding of the syndrome
Praveen Thumbikat 2
1 Department of Urology, Feinberg School of Medicine, Northwestern University, Chicago, United States
2 Department of Urology, Chicago, United States
There remains a limited understanding of the etiology of CPPS. Given the diffuse nature of the symptoms and the heterogeneous nature of the patient population this is not surprising. Of the information that exists it is possible that further subdividing the patient population based on certain inflammatory criteria might be useful in basic research on CPPS going forward. Our laboratory and others have begun to understand and regard CPPS as an underlying autoimmune defect that is exacerbated by damage to the prostate resulting in a chronic symptomology. Much of this robust information on the emergence of CPPS has come from research performed in murine models of the disease.
Various studies have sought to determine an association between CPPS and immune activation using samples from multiple sources including, expressed prostatic secretion (EPS), seminal plasma, semen and urine. These have shown not only an increase in the number of infiltrating cells (activated T and B cells, granulocytes and macrophages) ,  but also increased levels of the IL2 receptor ,  and specific inflammatory cytokines including IL1b, IL6, IL8, IgA and TNFa , , , . As yet investigators have failed to determine a specific prostate antigen responsible for driving the autoimmunity in patients but research has been successful in isolating Th1 T-cells specific to prostate specific antigen (PSA)  in the peripheral blood of CPPS patients in the absence of carcinogenesis . Auto-reactive CD4 T-cells have also been identified that respond to the seminal plasma of patients . Further investigation to narrow down specific antigens have revealed IgA antibodies against Ny-Co-7 and MAD-PRO-34, both prostate specific, in CPPS patients , . Taken together this data suggests that in humans there is some evidence of auto-reactivity against the prostate, which could mediate CPPS. This is supported histologically with one study showing an increase in the number of CD8+ T-cells in the prostate of patients compared to controls , . Studies from our laboratory using a high-throughput multiplex array for over 40 cytokines and chemokines has shown an increased level of IL7 expression in CPPS patients compared to controls, and demonstrated that increased levels of IL7 was also positively correlated with increases in patient reported symptoms . These findings were corroborated in the experimental autoimmune prostatitis (EAP) murine model where we identified prostate specific increases in IL7 expression . IL7 is a driver of T-cell differentiation and function , ,  and as such these findings also point to an underlying immune defect in CPPS patients.
Experimental autoimmune prostatitis
The experimental autoimmune prostatitis model (EAP) is a xenogenic mouse model of CPPS that is induced by a sub-cutaneous injection of a prostate homogenate or specific prostate proteins and an adjuvant , . The model was first investigated in Wistar rats and has since been adapted into alternative forms in mice . Early studies seemed to suggest that development of prostatitis was dependent on the prostate steroid binding protein (PSBP) which is was a candidate for a specific antigenic driver of pain as when used alone could induce CPPS-symptoms , , . Additional studies determined that other antigens within the prostate homogenate are also important. Depending on the adjuvant used there are multiple different inflammatory and adaptive immune responses that are mounted that account for associated symptoms. Our model of EAP uses a whole rat prostate homogenate and the Titermax adjuvant, which we have demonstrated results in increased number of Th17 cells in B6 and NOD animals , . Other models using CFA (complete freuds adjuvant) as an adjuvant, drive a more Th1-type (Il12p40 specifically) ,  response without the similar increase or necessity for Th17 cells, but have also been successful in driving development of chronic symptoms in mice . This use of different adjuvants to result in similar phenotypes is of particular interest and suggest that it maybe a defect in the ability of patients to control T-cell activation (Th1 or Th17 responses) by T-regulatory cells that is the underlying problem in patients rather than an excess of activation. This is further exemplified by the propensity for development of prostatitis in aged NOD mice compared to their B6 or Balb/c counterparts . The NOD mouse is used in diabetes research where T-cell immune activation against pancreatic B-islet cells has been demonstrated , . In the study of CPPS, NOD mice consistently develop pain responses that are significantly higher than their B6 counterparts in EAP models while also being uniquely susceptible to the development of chronic tactile allodynia in the CP1-induced model, see below . NOD mice have been shown to have genetic polymorphisms in two genes that regulate T-cell function, IDD3 and CTLA4 , both of which have specific roles in maturation and functioning of T-regulatory cells . This suggests that loss of T-regulatory cell function in these mice may account for the development of spontaneous prostatitis and prime the prostate immune microenvironment to development of chronic symptoms when activated.
Using the EAP model our laboratory demonstrated that two chemokines previously demonstrated to be associated with CPPS in humans, chemokine C-C motif ligands 2 and 3 (CCL2 and CCL3) were increased in prostate tissues during disease progression . CCL2, also known as MCP1 (monocyte chemotactic protein 1) and CCL3, also known as MIP1a (macrophage inflammation protein 1 alpha) have been associated with development of multiple autoimmune disorders including rheumatoid arthritis , , , , . In CPPS patients both chemokines were increased in EPS samples compared to controls but only MIP1a was positively correlated with increased symptoms severity . From the mouse model we demonstrated that CCL3 was increased only at later time-points (day 20) following EAP induction . This suggests that in patients as well as in mice that increased expression of certain chemokines may be dependent on when during disease course samples are collected and analyzed. Such differences may account for continuing difficulty in identifying a robust biomarker for CPPS as the immune microenvironment may be constantly shifting and changing.
One cell type in particular that has emerged as a potential major mediator of inflammation and development of centralized pain in CPPS, the mast cell. Data from both human and mouse studies have revealed a central role for these cells in maintenance of chronic symptoms , , , , , , , , . Mast cells are hematopoietic in nature and circulate in an immature form only differentiating fully once tissue resident. Such developmental processes are not unique to this cell type but do suggest some tissue specific cellular phenotype. These cells function as one of primary immune mediators to pathogenic infection and have multiple additional roles including tissue remodeling. Degranulation of mast cells upon activation is triggered by a variety of signals including the cytokine milieu, hormonal changes, physical changes and specific damage/pathogen-associated molecular patterns (D/PAMPs) . Degranulation results in release of numerous factors and damage response elements, such as serotonin, prostaglandins, histamine and tryptase. In human disease mast cells are associated with a variety of autoimmune disorders including RA , , , , , , where increased numbers of cells and associated tryptase has been shown at affected sites. Mast cells have been shown to interact directly with T-cells both pro-inflammatory, Th17 cells and T-regulatory cells via the OX40 ligand , .
EPS from CPPS patients compared to controls has also been shown to have an increased level of tryptase and an increased number of mast cells . Tryptase activates the protease-activated receptor 2 (PAR2), a member of the G-protein coupled receptor (GCPR) family that has known functions in pain and inflammation. The tryptase: PAR2 axis has been shown to be important for viseral pain and immune responses in ulcerative colitis and Crohn’s disease . Our laboratory has demonstrated significant increases in the levels of tryptase and carboxypeptidase A (CPA3) another mast cell released factor in EPS samples from patients, indicating increased mast cell activity in CPPS . Extending these findings into our murine EAP model we also demonstrated that PAR2 global knock-out mice are resistant to the development of pain upon EAP induction. Loss of PAR2 receptor expression appears to ablate MAPK/ERK signaling at the level of the dorsal root ganglion (DRG) associated with the prostate. We hypothesize that the mast cell might therefore mediate cross-talk between the immune response and the neurologic system in CPPS development . Inhibition of PAR2 receptor activity therapeutically, using a blocking antibody, ameliorates tactile allodynia in mice and we are currently implementing the use of mast cell stabilizers clinically as part of a small trial. Taken together these data suggest that immunological activation may enhance mast cell activity, which can serve to coordinate neurological interactions resulting in chronic pain .
Bacteria in CPPS
While there is mounting evidence for the role of the immune system in the etiology of CPPS it is by no means well defined. Underpinning this is the source of the initial prostate damage. Our studies and others are beginning to determine that certain bacteria may be our best hope to further understanding this syndrome. Although CPPS is distinguished from the other sub-categories of prostatitis by the absence of an associated bacterial infection, bacteria can readily be detected and isolated from both EPS and urine samples. Comparisons of the microbiome isolated from voided bladder (VB) samples between patients and control samples have been performed and have not, as yet, demonstrated significant shifts in the microbial ecology , . Currently as part of the MAPP project, research is underway that examines differences in the intestinal microbiome of patients versus controls. While these studies are useful from a therapeutic standpoint there is still a need to resolve deeper information into the etiology of the syndrome. A more robust approach from an etiological standpoint might be a deeper longitudinal characterization of microbiome shifts within the urogenital tract of patients to identify bacterial species that are prostate localized that are associated with CPPS symptoms over time. To this end our laboratory has focused on examining prostate localized bacterial isolates from CPPS patients to determine the role of specific human microbes in driving disease.
CP-1 (chronic-pain 1) is a prostate localized E. coli strain that was isolated from the EPS of a CPPS patient with active disease . Our laboratory has demonstrated that intra-urethral infection with this bacterial species can induce chronic tactile allodynia in NOD but not B6 mice. Immune responses to the bacterial infection skew towards a Th17 response and infiltration of leukocytes to the prostate and inflammation are sustained after bacterial clearance . We further demonstrate in this study that this immune activation and subsequent development of symptoms is transferable by adoptive transfer of ex vivo expanded T-cells that are skewed towards a Th17 phenotype . It is important to note that to date no specific role for Th17 cells has been identified from human cells and the current data is mainly correlative. The specificity of these data was further demonstrated by comparison with a cystitis associated pathogenic E. coli strain, NU14, which failed to mount similar responses and did not result in development of chronic pain. These strains are evolutionarily distinct CP1 belonging to UPEC group B1 while NU14 is a group B2 strain. Further analysis revealed that NU14 is equally capable of adhering to prostate epithelial cells but that CP1 is inherently more invasive . These studies underlined the bacterial specificity that we hypothesize to be very important in development and initiation of CPPS in humans. Furthermore this evidence also supports the theory that initial damage, such as a bacterial infection in certain genetic contexts can mount immune responses that fail to be controlled adequately resulting in development of chronic symptoms. The ability of the microbial ecology of the prostate to influence the immune microenvironment was examined further through our investigation on the potential of a commensal non-pathogenic bacterial strain, isolated from the prostate of a healthy man to control immune activation and ameliorate pain. Using our EAP model we demonstrate that intra-urethral instillation with a gram-positive Staphylococcus epidermidis species designated NPI (non-pain inducing), could reverse EAP-associated IL17 expression and significantly reduce tactile allodynia responses . NPI alone does not induce tactile allodynia in either B6 or NOD mice but is capable of colonizing prostate tissues in both animal backgrounds. More recently we are examining the effect of instillation of this bacteria in the context of an ongoing CP1 infection and are demonstrating that NPI colonization can prevent CP1-induced pain from developing. Taken together these findings demonstrate that specific bacteria from the prostate may have a role in initiation of CPPS in humans, that this can be maintained in the absence of an ongoing infection and most importantly that it can be reversed upon restoration of healthy immune: microbial interactions.
To further emphasize this point we are currently examining the potential of gram-positive bacterial species, isolated from the EPS of CPPS patients to induce pain in mice. In prostatitis diagnoses, including CPPS, gram-positive bacteria are usually deemed clinically insignificant and traditional uropathogens are thought to be gram-negative in nature. We have observed however that gram-positive species make up a large proportion of the bacterial content of the EPS and that when isolated from patient samples are capable of potentiating disease in a manner similar to CP1. Our initial studies suggest that intra-urethral instillation with three of these strains, S. epidermidis, E. faecalis, and S. hemolyticus, induces chronic tactile allodynia in NOD but not B6 animals. Furthermore characterization of the immune response of these animals to bacterial infection reveals that it is not mediated by Th17 cells but may involved NK-cell immune activation. These findings further support our hypothesis that it is loss of regulatory control of immune responses in these mice that result in CPPS-like symptom emergence rather than a particular flavor of adaptive immune response. This mirrors the seemingly conflicting evidence from the different EAP murine models of CPPS, which have shown opposing inflammatory processes to be dispensable and/or necessary for inflammation and pain development.
We have presented here the current understanding of the etiology of CPPS from a microbial and immunological perspective. From studies using both patient samples and murine models we postulate that it is the inter-play between bacteria and the immune system that is the major initiator of the syndrome and symptoms are then maintained owing to a host genetic defect in regulation of the adaptive immune response. Large patient-focused studies on the ongoing immune response throughout symptom maintenance and also the urinary microbiome changes in patients are necessary to further delineate this and more importantly to uncover potential therapeutics.
References Schaeffer AJ, Knauss JS, Landis JR, Propert KJ, Alexander RB, Litwin MS, Nickel JC, O'Leary MP, Nadler RB, Pontari MA, Shoskes DA, Zeitlin SI, Fowler JE Jr, Mazurick CA, Kusek JW, Nyberg LM; Chronic Prostatitis Collaborative Research Network Study Group. Leukocyte and bacterial counts do not correlate with severity of symptoms in men with chronic prostatitis: the National Institutes of Health Chronic Prostatitis Cohort Study. J Urol. 2002 Sep;168(3):1048-53. DOI: 10.1016/S0022-5347(05)64572-7
 Nickel JC, Alexander RB, Schaeffer AJ, Landis JR, Knauss JS, Propert KJ; Chronic Prostatitis Collaborative Research Network Study Group. Leukocytes and bacteria in men with chronic prostatitis/chronic pelvic pain syndrome compared to asymptomatic controls. J Urol. 2003 Sep;170(3):818-22. DOI: 10.1097/01.ju.0000082252.49374.e9
 John H, Barghorn A, Funke G, Sulser T, Hailemariam S, Hauri D, Joller-Jemelka H. Noninflammatory chronic pelvic pain syndrome: immunological study in blood, ejaculate and prostate tissue. Eur Urol. 2001 Jan;39(1):72-8. DOI: 10.1159/000052415
 John H, Maake C, Barghorn A, Zbinden R, Hauri D, Joller-Jemelka HI. Immunological alterations in the ejaculate of chronic prostatitis patients: clues for autoimmunity. Andrologia. 2003 Oct;35(5):294-9. DOI: 10.1111/j.1439-0272.2003.tb00860.x
 Penna G, Mondaini N, Amuchastegui S, Degli Innocenti S, Carini M, Giubilei G, Fibbi B, Colli E, Maggi M, Adorini L. Seminal plasma cytokines and chemokines in prostate inflammation: interleukin 8 as a predictive biomarker in chronic prostatitis/chronic pelvic pain syndrome and benign prostatic hyperplasia. Eur Urol. 2007 Feb;51(2):524-33; discussion 533. DOI: 10.1016/j.eururo.2006.07.016
 Motrich RD, Maccioni M, Ponce AA, Gatti GA, Oberti JP, Rivero VE. Pathogenic consequences in semen quality of an autoimmune response against the prostate gland: from animal models to human disease. J Immunol. 2006 Jul 15;177(2):957-67. DOI: 10.4049/jimmunol.177.2.957
 Ludwig M, Steltz C, Huwe P, Schäffer R, Altmannsberger M, Weidner W. Immunocytological analysis of leukocyte subpopulations in urine specimens before and after prostatic massage. Eur Urol. 2001 Mar;39(3):277-82. DOI: 10.1159/000052453
 Khadra A, Fletcher P, Luzzi G, Shattock R, Hay P. Interleukin-8 levels in seminal plasma in chronic prostatitis/chronic pelvic pain syndrome and nonspecific urethritis. BJU Int. 2006 May;97(5):1043-6. DOI: 10.1111/j.1464-410X.2006.06133.x
 Motrich RD, Maccioni M, Molina R, Tissera A, Olmedo J, Riera CM, Rivero VE. Presence of INFgamma-secreting lymphocytes specific to prostate antigens in a group of chronic prostatitis patients. Clin Immunol. 2005 Aug;116(2):149-57. DOI: 10.1016/j.clim.2005.03.011
 Ponniah S, Arah I, Alexander RB. PSA is a candidate self-antigen in autoimmune chronic prostatitis/chronic pelvic pain syndrome. Prostate. 2000 Jun 15;44(1):49-54. DOI: 10.1002/1097-0045(20000615)44:1<49::AID-PROS7>3.0.CO;2-7
 Alexander RB, Brady F, Leffell MS, Tsai V, Celis E. Specific T cell recognition of peptides derived from prostate-specific antigen in patients with prostate cancer. Urology. 1998 Jan;51(1):150-7. DOI: 10.1016/S0090-4295(97)00480-9
 Doble A, Walker MM, Harris JR, Taylor-Robinson D, Witherow RO. Intraprostatic antibody deposition in chronic abacterial prostatitis. Br J Urol. 1990 Jun;65(6):598-605.
 Batstone GR, Doble A, Gaston JS. Autoimmune T cell responses to seminal plasma in chronic pelvic pain syndrome (CPPS). Clin Exp Immunol. 2002 May;128(2):302-7. DOI: 10.1046/j.1365-2249.2002.01853.x
 Breser ML, Motrich RD, Sanchez LR, Mackern-Oberti JP, Rivero VE. Expression of CXCR3 on specific T cells is essential for homing to the prostate gland in an experimental model of chronic prostatitis/chronic pelvic pain syndrome. J Immunol. 2013 Apr;190(7):3121-33. DOI: 10.4049/jimmunol.1202482
 Murphy SF, Schaeffer AJ, Done J, Wong L, Bell-Cohn A, Roman K, Cashy J, Ohlhausen M, Thumbikat P. IL17 Mediates Pelvic Pain in Experimental Autoimmune Prostatitis (EAP). PLoS ONE. 2015;10(5):e0125623. DOI: 10.1371/journal.pone.0125623
 Zuvich RL, McCauley JL, Oksenberg JR, Sawcer SJ, De Jager PL; International Multiple Sclerosis Genetics ConsortiumAubin C, Cross AH, Piccio L, Aggarwal NT, Evans D, Hafler DA, Compston A, Hauser SL, Pericak-Vance MA, Haines JL. Genetic variation in the IL7RA/IL7 pathway increases multiple sclerosis susceptibility. Hum Genet. 2010 Mar;127(5):525-35. DOI: 10.1007/s00439-010-0789-4
 Heninger AK, Theil A, Wilhelm C, Petzold C, Huebel N, Kretschmer K, Bonifacio E, Monti P. IL-7 abrogates suppressive activity of human CD4+CD25+FOXP3+ regulatory T cells and allows expansion of alloreactive and autoreactive T cells. J Immunol. 2012 Dec;189(12):5649-58. DOI: 10.4049/jimmunol.1201286
 Miller CN, Hartigan-O'Connor DJ, Lee MS, Laidlaw G, Cornelissen IP, Matloubian M, Coughlin SR, McDonald DM, McCune JM. IL-7 production in murine lymphatic endothelial cells and induction in the setting of peripheral lymphopenia. Int Immunol. 2013 Aug;25(8):471-83. DOI: 10.1093/intimm/dxt012
 Motrich RD, Maccioni M, Riera CM, Rivero VE. Autoimmune prostatitis: state of the art. Scand J Immunol. 2007 Aug-Sep;66(2-3):217-27. DOI: 10.1111/j.1365-3083.2007.01971.x
 Rudick CN, Schaeffer AJ, Thumbikat P. Experimental autoimmune prostatitis induces chronic pelvic pain. Am J Physiol Regul Integr Comp Physiol. 2008 Apr;294(4):R1268-75. DOI: 10.1152/ajpregu.00836.2007
 Rivero VE, Iribarren P, Riera CM. Mast cells in accessory glands of experimentally induced prostatitis in male Wistar rats. Clin Immunol Immunopathol. 1995 Mar;74(3):236-42. DOI: 10.1006/clin.1995.1035
 Maccioni M, Rivero VE, Riera CM. Prostatein (or rat prostatic steroid binding protein) is a major autoantigen in experimental autoimmune prostatitis. Clin Exp Immunol. 1998 May;112(2):159-65. DOI: 10.1046/j.1365-2249.1998.00588.x
 Rivero V, Carnaud C, Riera CM. Prostatein or steroid binding protein (PSBP) induces experimental autoimmune prostatitis (EAP) in NOD mice. Clin Immunol. 2002 Nov;105(2):176-84. DOI: 10.1006/clim.2002.5281
 Penna G, Amuchastegui S, Cossetti C, Aquilano F, Mariani R, Giarratana N, De Carli E, Fibbi B, Adorini L. Spontaneous and prostatic steroid binding protein peptide-induced autoimmune prostatitis in the nonobese diabetic mouse. J Immunol. 2007 Aug 1;179(3):1559-67. DOI: 10.4049/jimmunol.179.3.1559
 Motrich RD, Breser ML, Sánchez LR, Godoy GJ, Prinz I, Rivero VE. IL-17 is not essential for inflammation and chronic pelvic pain development in an experimental model of chronic prostatitis/chronic pelvic pain syndrome. Pain. 2016 Mar;157(3):585-97. DOI: 10.1097/j.pain.0000000000000405
 Breser ML, Salazar FC, Rivero VE, Motrich RD. Immunological Mechanisms Underlying Chronic Pelvic Pain and Prostate Inflammation in Chronic Pelvic Pain Syndrome. Front Immunol. 2017;8:898. DOI: 10.3389/fimmu.2017.00898
 Breser ML, Motrich RD, Sanchez LR, Rivero VE. Chronic Pelvic Pain Development and Prostate Inflammation in Strains of Mice With Different Susceptibility to Experimental Autoimmune Prostatitis. Prostate. 2017 Jan;77(1):94-104. DOI: 10.1002/pros.23252
 Louvet C, Szot GL, Lang J, Lee MR, Martinier N, Bollag G, Zhu S, Weiss A, Bluestone JA. Tyrosine kinase inhibitors reverse type 1 diabetes in nonobese diabetic mice. Proc Natl Acad Sci USA. 2008 Dec;105(48):18895-900. DOI: 10.1073/pnas.0810246105
 Augstein P, Elefanty AG, Allison J, Harrison LC. Apoptosis and beta-cell destruction in pancreatic islets of NOD mice with spontaneous and cyclophosphamide-accelerated diabetes. Diabetologia. 1998 Nov;41(11):1381-8. DOI: 10.1007/s001250051080
 Rudick CN, Berry RE, Johnson JR, Johnston B, Klumpp DJ, Schaeffer AJ, Thumbikat P. Uropathogenic Escherichia coli induces chronic pelvic pain. Infect Immun. 2011 Feb;79(2):628-35. DOI: 10.1128/IAI.00910-10
 Lundholm M, Motta V, Löfgren-Burström A, Duarte N, Bergman ML, Mayans S, Holmberg D. Defective induction of CTLA-4 in the NOD mouse is controlled by the NOD allele of Idd3/IL-2 and a novel locus (Ctex) telomeric on chromosome 1. Diabetes. 2006 Feb;55(2):538-44. DOI: 10.2337/diabetes.55.02.06.db05-1240
 Ceeraz S, Nowak EC, Noelle RJ. B7 family checkpoint regulators in immune regulation and disease. Trends Immunol. 2013 Nov;34(11):556-63. DOI: 10.1016/j.it.2013.07.003
 Quick ML, Mukherjee S, Rudick CN, Done JD, Schaeffer AJ, Thumbikat P. CCL2 and CCL3 are essential mediators of pelvic pain in experimental autoimmune prostatitis. Am J Physiol Regul Integr Comp Physiol. 2012 Sep;303(6):R580-9. DOI: 10.1152/ajpregu.00240.2012
 Koch AE, Kunkel SL, Harlow LA, Johnson B, Evanoff HL, Haines GK, Burdick MD, Pope RM, Strieter RM. Enhanced production of monocyte chemoattractant protein-1 in rheumatoid arthritis. J Clin Invest. 1992 Sep;90(3):772-9. DOI: 10.1172/JCI115950
 Koch AE, Kunkel SL, Pearce WH, Shah MR, Parikh D, Evanoff HL, Haines GK, Burdick MD, Strieter RM. Enhanced production of the chemotactic cytokines interleukin-8 and monocyte chemoattractant protein-1 in human abdominal aortic aneurysms. Am J Pathol. 1993 May;142(5):1423-31.
 Ikeda U, Matsui K, Murakami Y, Shimada K. Monocyte chemoattractant protein-1 and coronary artery disease. Clin Cardiol. 2002 Apr;25(4):143-7. DOI: 10.1002/clc.4960250403
 Van Steenwinckel J, Reaux-Le Goazigo A, Pommier B, Mauborgne A, Dansereau MA, Kitabgi P, Sarret P, Pohl M, Mélik Parsadaniantz S. CCL2 released from neuronal synaptic vesicles in the spinal cord is a major mediator of local inflammation and pain after peripheral nerve injury. J Neurosci. 2011 Apr;31(15):5865-75. DOI: 10.1523/JNEUROSCI.5986-10.2011
 Zhang N, Inan S, Inan S, Cowan A, Sun R, Wang JM, Rogers TJ, Caterina M, Oppenheim JJ. A proinflammatory chemokine, CCL3, sensitizes the heat- and capsaicin-gated ion channel TRPV1. Proc Natl Acad Sci USA. 2005 Mar;102(12):4536-41. DOI: 10.1073/pnas.0406030102
 Desireddi NV, Campbell PL, Stern JA, Sobkoviak R, Chuai S, Shahrara S, Thumbikat P, Pope RM, Landis JR, Koch AE, Schaeffer AJ. Monocyte chemoattractant protein-1 and macrophage inflammatory protein-1alpha as possible biomarkers for the chronic pelvic pain syndrome. J Urol. 2008 May;179(5):1857-61; discussion 1861-2. DOI: 10.1016/j.juro.2008.01.028
 Lee DM, Friend DS, Gurish MF, Benoist C, Mathis D, Brenner MB. Mast cells: a cellular link between autoantibodies and inflammatory arthritis. Science. 2002 Sep;297(5587):1689-92. DOI: 10.1126/science.1073176
 Theoharides TC, Cochrane DE. Critical role of mast cells in inflammatory diseases and the effect of acute stress. J Neuroimmunol. 2004 Jan;146(1-2):1-12.
 Metz M, Maurer M. Mast cells--key effector cells in immune responses. Trends Immunol. 2007 May;28(5):234-41. DOI: 10.1016/j.it.2007.03.003
 Rao KN, Brown MA. Mast cells: multifaceted immune cells with diverse roles in health and disease. Ann N Y Acad Sci. 2008 Nov;1143:83-104. DOI: 10.1196/annals.1443.023
 Sayed BA, Christy A, Quirion MR, Brown MA. The master switch: the role of mast cells in autoimmunity and tolerance. Annu Rev Immunol. 2008;26:705-39. DOI: 10.1146/annurev.immunol.26.021607.090320
 Shin K, Nigrovic PA, Crish J, Boilard E, McNeil HP, Larabee KS, Adachi R, Gurish MF, Gobezie R, Stevens RL, Lee DM. Mast cells contribute to autoimmune inflammatory arthritis via their tryptase/heparin complexes. J Immunol. 2009 Jan 1;182(1):647-56. DOI: 10.4049/jimmunol.182.1.647
 de Vries VC, Wasiuk A, Bennett KA, Benson MJ, Elgueta R, Waldschmidt TJ, Noelle RJ. Mast cell degranulation breaks peripheral tolerance. Am J Transplant. 2009 Oct;9(10):2270-80. DOI: 10.1111/j.1600-6143.2009.02755.x
 Done JD, Rudick CN, Quick ML, Schaeffer AJ, Thumbikat P. Role of mast cells in male chronic pelvic pain. J Urol. 2012 Apr;187(4):1473-82. DOI: 10.1016/j.juro.2011.11.116
 Johnson D, Seeldrayers PA, Weiner HL. The role of mast cells in demyelination. 1. Myelin proteins are degraded by mast cell proteases and myelin basic protein and P2 can stimulate mast cell degranulation. Brain Res. 1988 Mar 15;444(1):195-8. DOI: 10.1016/0006-8993(88)90929-8
 Sandler C, Lindstedt KA, Joutsiniemi S, Lappalainen J, Juutilainen T, Kolah J, Kovanen PT, Eklund KK. Selective activation of mast cells in rheumatoid synovial tissue results in production of TNF-alpha, IL-1beta and IL-1Ra. Inflamm Res. 2007 Jun;56(6):230-9. DOI: 10.1007/s00011-007-6135-1
 Hueber AJ, Asquith DL, Miller AM, Reilly J, Kerr S, Leipe J, Melendez AJ, McInnes IB. Mast cells express IL-17A in rheumatoid arthritis synovium. J Immunol. 2010 Apr;184(7):3336-40. DOI: 10.4049/jimmunol.0903566
 Sawamukai N, Yukawa S, Saito K, Nakayamada S, Kambayashi T, Tanaka Y. Mast cell-derived tryptase inhibits apoptosis of human rheumatoid synovial fibroblasts via rho-mediated signaling. Arthritis Rheum. 2010 Apr;62(4):952-9. DOI: 10.1002/art.27331
 Gri G, Piconese S, Frossi B, Manfroi V, Merluzzi S, Tripodo C, Viola A, Odom S, Rivera J, Colombo MP, Pucillo CE. CD4+CD25+ regulatory T cells suppress mast cell degranulation and allergic responses through OX40-OX40L interaction. Immunity. 2008 Nov;29(5):771-81. DOI: 10.1016/j.immuni.2008.08.018
 Roman K, Done JD, Schaeffer AJ, Murphy SF, Thumbikat P. Tryptase-PAR2 axis in experimental autoimmune prostatitis, a model for chronic pelvic pain syndrome. Pain. 2014 Jul;155(7):1328-38. DOI: 10.1016/j.pain.2014.04.009
 Cenac N, Andrews CN, Holzhausen M, Chapman K, Cottrell G, Andrade-Gordon P, Steinhoff M, Barbara G, Beck P, Bunnett NW, Sharkey KA, Ferraz JG, Shaffer E, Vergnolle N. Role for protease activity in visceral pain in irritable bowel syndrome. J Clin Invest. 2007 Mar;117(3):636-47. DOI: 10.1172/JCI29255
 Lewis DA, Brown R, Williams J, White P, Jacobson SK, Marchesi JR, Drake MJ. The human urinary microbiome; bacterial DNA in voided urine of asymptomatic adults. Front Cell Infect Microbiol. 2013 Aug 15;3:41. DOI: 10.3389/fcimb.2013.00041
 Nickel JC, Stephens A, Landis JR, Chen J, Mullins C, van Bokhoven A, Lucia MS, Melton-Kreft R, Ehrlich GD; MAPP Research Network. Search for Microorganisms in Men with Urologic Chronic Pelvic Pain Syndrome: A Culture-Independent Analysis in the MAPP Research Network. J Urol. 2015 Jul;194(1):127-35. DOI: 10.1016/j.juro.2015.01.037
 Quick ML, Wong L, Mukherjee S, Done JD, Schaeffer AJ, Thumbikat P. Th1-Th17 cells contribute to the development of uropathogenic Escherichia coli-induced chronic pelvic pain. PLoS ONE. 2013;8(4):e60987. DOI: 10.1371/journal.pone.0060987
 Rudick CN, Billips BK, Pavlov VI, Yaggie RE, Schaeffer AJ, Klumpp DJ. Host-pathogen interactions mediating pain of urinary tract infection. J Infect Dis. 2010 Apr;201(8):1240-9. DOI: 10.1086/651275
 Murphy SF, Schaeffer AJ, Done JD, Quick ML, Acar U, Thumbikat P. Commensal bacterial modulation of the host immune response to ameliorate pain in a murine model of chronic prostatitis. Pain. 2017 Aug;158(8):1517-1527. DOI: 10.1097/j.pain.0000000000000944