Nickoo Merati, MSc1; Sriraam Sivachandran1; Abdulhadi Jfri, MD1; Moshe Ben-Shoshan, MD2; Donald C. Vinh, MD, FRCPC2,3; Gizelle Popradi, MD, FRCPC4; Ivan V. Litvinov, MD, PhD, FRCPC1

1Division of Dermatology, McGill University Health Centre, Montréal, QC, Canada
2Division of Allergy and Immunology, McGill University Health Centre, Montréal, QC, Canada
3Division of Infectious Diseases, McGill University Health Centre, Montréal, QC, Canada
4Division of Hematology, McGill University Health Centre, Montréal, QC, Canada

Authorship statement:
N. Merati: reviewed literature, prepared the manuscript;
S. Sivachandran: addressed reviewer comments, reviewed literature and co-wrote the paper;
A. Jfri: contributed to the review of literature and preparation of the manuscript;
M. Ben-Shoshan: contributed to the review of literature and preparation of the manuscript;
D. C. Vinh: contributed to the review of literature and preparation of the manuscript;
G. Popradi: contributed to the review of literature and preparation of the manuscript;
I. V. Litvinov: supervised the study, reviewed literature and co-wrote the paper.

Conflict of interest:
The authors declare no potential conflicts of interest with respect to the research, authorship, and/or publication of this article. Funding: None.

Warts, Hypogammaglobulinemia, Infections and Myelokathexis (WHIM) is a primary immunodeficiency syndrome. Patients with WHIM syndrome are more susceptible to human papillomavirus (HPV) infections and commonly present to a dermatologist with recalcitrant to treatment warts. Other cardinal features of WHIM syndrome include recurrent sinopulmonary bacterial infections, neutropenia/lymphopenia, low levels of immunoglobulins (IgG, IgA, IgM) and myelokathexis. Research demonstrated that truncating gain-of-function mutations of the C-X-C chemokine receptor type 4 gene (CXCR4) are responsible for this disease. Plerixafor, a specific small molecule antagonist of CXCR4, is currently used for peripheral blood hematopoietic stem cell (HSC) mobilization in stem cell transplant recipients. It has recently shown promise for the treatment of WHIM syndrome in phase I/II clinical trials. In this paper we review the emerging patient clinical data for this medication and highlight the role of CXCR4 in other important skin diseases including keratinocyte carcinomas, psoriasis and cutaneous T-cell lymphoma.

Key Words:
plerixafor, warts, hypogammaglobulinemia, infections and myelokathexis (WHIM) syndrome, C-X-C chemokine receptor type 4 (CXCR4), stromal cell-derived factor-1 (SDF-1), CXCL12, recalcitrant warts


Warts, Hypogammaglobulinemia, Infections and Myelokathexis (WHIM) is a primary immunodeficiency syndrome. As the name suggests, patients with WHIM syndrome are more susceptible to human papillomavirus (HPV),1 which can cause warts and potentially lead to squamous cell carcinomas; hypogammaglobulinemia; recurrent bacterial infections, such as otitis media, cellulitis, pneumonia and sinusitis; and bone marrow myelokathexis, characterized by a retention and apoptosis of mature neutrophils resulting neutropenia. The laboratory manifestations include severe neutropenia, significant lymphopenia, leukopenia, and monocytopenia.

WHIM syndrome is a rare disease with ~70 cases reported in the medical literature to date.2,3 In 2003, the cause of WHIM syndrome was traced to a heterozygous, truncating gain-of-function mutations of the C-X-C chemokine receptor type 4 gene (CXCR4) on chromosome 2 (2q22.1), resulting in a hyperactive signalling of this G-protein coupled receptor. While not yet fully deciphered, it is postulated that increased CXCR4 receptor activity upon binding of its cognate stromal cell-derived factor-1 (SDF-1; also known as CXCL12) ligand, prevents the release of mature neutrophils and promotes their apoptosis in the marrow (Figure 1A-B). This cellular mechanism drives myelokathexis that is observed clinically in affected patients. The CXCR4 mutations are inherited in autosomal dominant manner. Some patients with WHIM syndrome, however, do not have a detectable CXCR4 gene mutation, suggesting that mutations in other genes may be involved.

Plerixafor on a WHIM – Promise or Fantasy of a New CXCR4 Inhibitor for This Rare, but Important Syndrome? - image
Figure 1A. The chemokine stromal cell-derived factor-1 (SDF-1) binds to the transmembrane CXCR4 receptor (a G protein-coupled transmembrane protein) in the bone marrow. CXCR4 regulates the mobilization of neutrophils and lymphocytes from bone marrow by first blocking their release; as the neutrophils mature, SDF-1-CXCR4 is internalized and hence turned off, allowing for the release of neutrophils and lymphocytes and effective mobilization into the blood stream.
Plerixafor on a WHIM – Promise or Fantasy of a New CXCR4 Inhibitor for This Rare, but Important Syndrome? - image
Figure 1B. A gain-of-function mutation in patients with the WHIM syndrome leads to inappropriate hyperactivity of the CXCR4 receptor and failed internalization, leading to a prolonged retention of neutrophils and lymphocytes in the bone marrow leading to myelokathexis and immunodeficiency.


The diagnosis of WHIM syndrome relies on identifying the clinical features, a detailed patient medical history, family history, and genetic testing. In particular, patients with recalcitrant HPV warts and recurrent sinopulmonary bacterial infections should be further evaluated with a complete blood count and differential (to detect neutropenia and lymphopenia), and immunoglobulins (IgG, IgA, IgM) to detect hypogammaglobulinemia. Family history may reveal vertical transmission in autosomal dominant fashion, if more than one generation is affected. A bone marrow biopsy may be performed to detect myelokathexis.

While potential molecular mechanisms have been elucidated, the effectiveness of standard WHIM syndrome therapies is variable and more targeted therapies are urgently needed. Until recently most patients were managed using a combination of granulocyte colony stimulating factor (G-CSF), skin-directed treatments of warts, prophylactic antibiotics, and intravenous gamma globulin (IVIG) therapy.

In 2009, a group of researchers sought to therapeutically target the CXCR4 receptor. Plerixafor, a specific small molecule antagonist of CXCR4 (Figure 1C), was originally licensed by the United States Food and Drug Administration for peripheral blood hematopoietic stem cell (HSC) mobilization in stem cell transplant recipients. Plerixafor (Mozobil®) has been used in Canada and in the United States since 2012.4 Notably, a novel application for this medication has been defined, where McDermott and colleagues conducted a phase I clinical trial (NCT00967785) using plerixafor as a treatment for 20 patients with WHIM syndrome.3,5-7 They found that 9 patients who received low-dose plerixafor safely mobilized neutrophils and had an improvement in all other leukocyte subsets.3,5-7 A follow-up study in 3 of the participants demonstrated sustained responses for at least 6 months using a dosage of 0.02-0.04 mg/kg/day subcutaneously with no new warts developing and regression of old warts. In 2014, these promising results led the investigators to conduct a phase III randomized, double-blinded, crossover trial (NCT02231879) to establish safety and efficacy of plerixafor compared to standard G-CSF treatment in patients aged 10-75 years with WHIM syndrome. Nineteen patients were randomized to 1 year of G-CSF and 1 year of plerixafor using a crossover design, allowing direct comparison of infection severity during treatment with both agents. Doses were personalized to each patient’s neutrophil response. Study participants had a clinical diagnosis of WHIM syndrome and were proven to have a heterozygous mutation in the CXRC4 gene.8 While the trial is currently ongoing, some early data is beginning to emerge from this group.

Plerixafor on a WHIM – Promise or Fantasy of a New CXCR4 Inhibitor for This Rare, but Important Syndrome? - image
Figure 1C. Plerixafor binds to CXCR4 receptor in the bone marrow and blocks the SDF-1-CXCR4 interaction, allowing for normal mobilization of neutrophils and lymphocytes into the blood stream.

During recruitment, McDermott and colleagues identified 3 patients who were ineligible to participate in the larger study as they could not receive G-CSF. The researchers began a concomitant study with these patients, treating them with plerixafor (according to their phase 1 protocol) for 20-50 months. These findings have been published in the New England Journal of Medicine. McDermott et al. reported improvement in all 3 patients’ white cells counts, platelet counts and hemoglobin levels.9 In 2 out of 3 patients these results were observed after discontinuing G-CSF, which was deemed ineffective. Bone marrow biopsies revealed marked amelioration of severe pre-treatment myelofibrosis and myelokathexis in 2 of the patients after using plerixafor for 24 and 52 months.9 With the adjunct use of imiquimod in 2 of the patients and double HPV vaccination in 1 of the patients, HPV-associated wart burden improved noticeably on the hands, feet and genitals. Mixed results though were obtained with HPV-associated tumors, which were managed with debulking and surgery. Susceptibility to infections and inflammation also differed amongst the patients; however, 1 patient was noted to have a marked reduction in infection frequency compared to his pre-treatment baseline, and a significantly higher quality of life, where he was able to exercise, enjoy the outdoors and work without being fearful of recurrent infections and hospitalizations.

The results of these case studies are exciting and suggest that clinicians and patients with WHIM syndrome might expect favorable clinical outcomes with plerixafor. However, as with any new therapeutic indication, we must await the efficacy and safety results of the phase III trial, especially considering that chronic leukocyte mobilization from bone marrow with plerixafor could result in as of yet undescribed cumulative toxicities.10,11 It will also be crucial to determine the effect of plerixafor on immune cell function including mitogen induced T-cell proliferation and T-cell dependent humoral immunity.

Also, another phase II/III clinical trial (NCT03005327) is being conducted by researchers in Florida, investigating the use of mavorixafor (X4P-001), a different small molecule targeting hyperactive CXCR4 receptor.12 Funded by X4 Pharmaceuticals, researchers have demonstrated positive preliminary clinical findings using mavorixafor in a group of 6 WHIM patients, showing increased white cell counts and improved clinical outcomes. Unlike plerixafor, mavorixafor is administered orally.13 Additional phase II results published on 8 patients in Blood demonstrated that this medication was well tolerated by patients (with no treatment-related serious adverse events) using a dose of 400 mg daily and led to neutrophil mobilization, reduced infection rates and reduction in the number of warts.14


Emerging molecular experimental and clinical data suggests that CXCR4 may play an important role in other skin and systemic diseases including mycosis fungoides/Sézary syndrome, neurofibromatosis type 1 tumors, allergic reactions, Waldenström macroglobulinemia, melanoma, non-melanoma skin cancers and other solid tumors (Table 1). Hence, ability to target the CXCR4 may improve our abilities to treat a number of these diseases in the not-too-distant future. In fact, a number of trials using mavorixafor are underway evaluating efficacy in the treatment of Waldenström macroglobulinemia, renal cell carcinoma and other cancers, where CXCR4 inhibitors are actively being evaluated as cancer immunotherapy treatments.15,16

DiseaseProposed Involvement of CXCR4References
1.Sézary syndrome/cutaneous T-cell lymphomaLymphocyte skin homing may involve CXCR4 signaling. The CXCR4 chemokine receptor may play a role in homing of malignant T lymphocytes in mycosis fungoides and Sézary syndrome. CD26 (a dipeptidylpeptidase) cleaves and inactivates SDF-1 (a CXCR4 ligand) produced by stromal cells and fibroblasts in the dermis. The loss of CD26 on Sézary cells may increase their ability to migrate to and/or survive in the skin.17, 18
2.Skin warts and human papillomavirus (HPV)-related diseaseObserved in 61% of long-term WHIM syndrome patients with CXCR4 mutations. Expression of CXCR41013 (a WHIM-associated CXCR4 gain of function mutation) promotes stabilization of HPV oncoproteins. Thus, hyperactive CXCR4 could be an important facilitator in HPV-driven carcinogenesis.19, 20
3.Squamous cell carcinoma (SCC)Drugs that inhibit SDF-1 induced endocytosis of CXCR4 can suppresses cutaneous SCC cell migration. The SDF-1/CXCR4 signaling may also be involved in the establishment of lymph node metastasis in oral SCC, via activation of both ERK1/2 and Akt/PKB induced by Src family kinases. Analysis of chemokine receptor expression showed upregulation of CXCR4 in potentially metastatic non‐melanoma skin cancers and invasive oral SCCs.21-23
4.Basal cell carcinoma (BCC)CXCR4 expression may play a critical role in tumor progression and angiogenesis of certain subtypes of BCC with more aggressive phenotype. Functional blockade of CXCR4 signaling could be a potential therapeutic strategy for these tumors.24
5.Neurofibromatosis type 1 (NF1)CXCR4 gene expression increased 3- to 120-fold and SDF-1 gene expression increased 33- to 512-fold in NF1 tumors.25
6.MelanomaMelanoma cells use CXCR4 and CCR10 to enhance cell survival in the face of immune-mediated attack.26
7.Waldenstrom macroglobulinemia (WM)Whole genome sequencing in WM patients found that 27% had WHIM syndrome-like mutations in the CXCR4 gene.27
8.Allergic and eosinophil-related responsesGlucocorticoids were found to significantly upregulate CXCR4 expression in eosinophils. It was suggested that upregulation of CXCR4 may mediate the antiallergic properties of the glucocorticoid therapy by sequestering eosinophils from the circulation to non-inflamed extravascular tissues.28
9.Rheumatoid arthritisThe SDF-1/CXCR4 ligand-receptor signaling is likely playing an important functional role in T-cell accumulation and positioning within the diseased synovium in rheumatoid arthritis patients.29
10.PsoriasisElevated mRNA levels of both SDF-1 and CXCR4 have been found in lesional psoriatic skin.30
11.Breast, kidney and other solid tumorsBy quantitative RT-PCR, immunohistochemistry, and flow cytometric analysis CXCR4 was found to be highly expressed in primary and metastatic human breast cancer cells, but not in normal mammary tissues. RT-PCR detected peak expression levels of SDF-1 in lymph nodes, lung, liver, and bone marrow.31 This chemokine, as well as lung and liver extracts, induce directional migration of breast cancer cells in vitro, which can be blocked by specific CXCR4 antibodies. Histologic and RT-PCR analyses demonstrated that metastasis of breast cancer cells grown in mice could be significantly decreased by treatment with anti-CXCR4 antibodies.31 In kidney, von Hippel-Lindau tumor suppressor protein (VHL) decreases CXCR4 expression (by targeting hypoxia-inducible factor [HIF1-α] for degradation, a known transcriptional transactivator of CXCR4).32 Mutations in the VHL gene in clear cell carcinomas correlated with strong expression of CXCR4 and poor cancer-specific survival.32 Hence, high CXCR4 expression may promote cancer cell tendency to metastasize to specific organs.31, 32
12.Response to West Nile virus (WNV) infectionIn mouse models of WNV encephalitis the downregulation of the beta isoform of SDF-1 (CXCL12) and a corresponding decrease in perivascular T cells and an increase in parenchymal T cells was observed. Treatment with a continuously administered CXCR4 antagonist increased the survival of WNV-infected mice and produced a reduction in WNV burden in the brain. CXCR4 antagonism enhanced T-cell penetration into the brain after WNV encephalitis, increased virus-specific CD8+ T-cell interaction with infected cells and decreased glial cell activation.33
Table 1: Summary of studies documenting the importance of CXCR4 signaling in a number of important skin diseases and other
medical conditions.


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  21. Gong T, Yu Y, Yang B, et al. Celecoxib suppresses cutaneous squamous-cell carcinoma cell migration via inhibition of SDF1-induced endocytosis of CXCR4. Onco Targets Ther. 2018 Nov 12;11:8063-71.

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  26. Kakinuma T, Hwang ST. Chemokines, chemokine receptors, and cancer metastasis. J Leukoc Biol. 2006 Apr;79(4):639-51.

  27. Hunter ZR, Xu L, Yang G, et al. The genomic landscape of Waldenstrom macroglobulinemia is characterized by highly recurring MYD88 and WHIMlike CXCR4 mutations, and small somatic deletions associated with B-cell lymphomagenesis. Blood. 2014 Mar 13;123(11):1637-46.

  28. Nagase H, Miyamasu M, Yamaguchi M, et al. Glucocorticoids preferentially upregulate functional CXCR4 expression in eosinophils. J Allergy Clin Immunol. 2000 Dec;106(6):1132-9.

  29. Bradfield PF, Amft N, Vernon-Wilson E, et al. Rheumatoid fibroblastlike synoviocytes overexpress the chemokine stromal cell-derived factor 1 (CXCL12), which supports distinct patterns and rates of CD4+ and CD8+ T cell migration within synovial tissue. Arthritis Rheum. 2003 Sep;48(9):2472-82.

  30. Suarez-Farinas M, Fuentes-Duculan J, Lowes MA, et al. Resolved psoriasis lesions retain expression of a subset of disease-related genes. J Invest Dermatol. 2011 Feb;131(2):391-400.

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  33. McCandless EE, Zhang B, Diamond MS, et al. CXCR4 antagonism increases T cell trafficking in the central nervous system and improves survival from West Nile virus encephalitis. Proc Natl Acad Sci U S A. 2008 Aug 12;105(32):11270-5.

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