**2.2 Circulatory immune cells**

Although the distribution of peripheral blood cell populations can be a valuable indicator for establishing the patient's immune status and ability to develop an effective defense against pathogenic factors, circulatory immune cells have been less studied in Ps as compared to tissue-resident immune cells. Due to their important role in the pathogenesis of Ps, lymphocyte populations and subpopulations have been the most studied circulating immune cells. Tissue immune cells are mirrored by the circulatory ones as there is a constant circulation between these sites (**Figure 2**).

In the peripheral blood of Ps' patients, an increased T-CD4<sup>+</sup> cells activation and an imbalance in the Th1/Th2 ratio, with high Th1 and low Th2 phenotypes have been reported [63]. Also, higher levels of IFN-γ, IL-2, and IL-10, decreased concentration of IL-4, and increased expression of T-bet mRNA (Th1-specific transcription factor) sustain that Ps is predominantly a Th1-mediated disease [64]. In the early stages of Ps, the patients have increased IFN-γ expression, while patients in the chronic stage have high levels of IL-10. All these findings suggest a possible shift from Th1 to Th2 response in order to down-regulate the inflammatory response [65].

In a recent study regarding the immunophenotyping of T cells in Ps' peripheral blood, increased Th1/Th17 cells and decreased Th2/Tregs cells were reported. The percentages

#### **Figure 2.**

*Immune cells like activated dermal DCs can circulate through the lymphatic system into lymph nodes where DCs activate various T cell subpopulations that can travel into the skin and contribute to the activation of PDCs; the activation of cells within the skin would trigger the chemotaxis of monocytes and neutrophils into the skin and increase the inflammatory status of the psoriatic lesion. Created with BioRender.com.*

#### *Immune Markers in Psoriasis DOI: http://dx.doi.org/10.5772/intechopen.102567*

of Th1/Th17 cells were positively correlated with disease severity (PASI score) [63]. A positive correlation has also been reported between elevated levels of IL-21+ Th17 cells and IL-21 found in peripheral blood of Ps patients with the severity of the disease. The study also has shown that IL-21 can promote the differentiation into the Th17 subset, and recommends IL-21 as a potential immune marker [66].

Another T-CD4<sup>+</sup> cell subset involved in Ps' pathogenesis is Th22. Luan et al. reported elevated levels of circulating Th22, Tc22, and IL-22 in patients showing a positive correlation between Th22, IL-22, and PASI score. Nevertheless, no correlation was observed between circulating Tc22 and PASI score [67].

The studies regarding Tregs cells distribution in peripheral blood are controversial. Thus, some authors report a low frequency of circulating Tregs in Ps [68], others observed high percentages of Tregs correlated with PASI scores in moderate to severe Ps [69], and others show no differences compared to healthy volunteers [70]. Although the relevance of Tregs distribution in the periphery remains unclear, the decreased suppressive function and an altered Th17/Tregs balance contribute to the exacerbation of Ps.

T follicular helper (Tfh) cells are a specialized subset of T-CD4+ cells expressing increased levels of CXCR5, inducible T cell costimulatory (ICOS), programmed death protein-1 (PD-1), and the transcription factors B cell lymphoma 6 (Bcl-6). This sub-population actively secrets high levels of IL-21, IL-17, and IFN- [71]. Tfh cells are activated in Ps and identifying a higher percentage of circulating Tfh17 (CXCR3-CCR6+ phenotype) correlates with disease' severity. Thus, the frequency of circulating Tfh cells and the secretion of cytokines are significantly decreased after one month of treatment. All these findings indicate that activated circulating Tfh cells are involved in Ps pathogenesis and can constitute a potential therapeutic target for psoriatic disease [72].

In contrast to T-CD4+ cells, considered the key subset of pathogenic T lymphocytes, circulating T-CD8+ cells have been less studied and characterized. The frequency of circulating T-CD8+ cells which express cutaneous lymphocyte antigen (CLA) is higher in Ps patients compared to healthy individuals and is strongly correlated with PASI score [73]. Colombo et al. evaluated circulating IL17+ /IFN-γ<sup>+</sup> /IL-17/ IL-22+ T-CD8 cells in Ps, psoriatic arthritis (PsA), and rheumatoid arthritis (RA), and reported high levels of IFN-γ<sup>+</sup> T-CD8 cells in PsA compared to Ps. A significant correlation between the extent and severity of Ps and the frequency of circulating IL-17+ T-CD8 cells was as well reported [74].

Although B cells play also an important role in skin inflammation, their distribution in peripheral blood has been poorly studied in Ps. Lu et al. found upregulated percentages of CD19+ B cells in peripheral blood mononuclear cells (PBMCs) of Ps patients, which were positively correlated with PASI score. The authors also investigated the expression of CD40, CD44, CD80, CD86, and CD11b on B cells in 4 clinical types of Ps and showed that the expressions of these activation markers are different in various types of Ps [75]. Other studies reported decreased circulating IL-10 producing Bregs cells in Ps, negatively correlated with Th1, Th17 cells, and IFNγ<sup>+</sup> and IL-17<sup>+</sup> NKT cells. During apremilast treatment, these values increased and were correlated with the clinical response [76, 77].

NK cells, known for their anti-viral and anti-tumoral functions, were found in the inflammatory infiltrate of the psoriatic lesions. Even though Ps is a skin disorder, there have been reported changes in circulating NK cells. The number of circulating NK cells is reduced in Ps patients. The levels of circulating IFN-γ and TNF-α are similar to healthy controls [78]. The low levels of NK cells in Ps patients peripheral blood, correlated with a lower frequency of cells expressing NK specific markers (CD56, CD16, CD94, CD158a) but no correlation with the severity of the disease [79]. NK cells had

increased expression of the apoptosis-associated Fas receptor and lower expression of CD94 and NKG2A. In addition to its ability to induce apoptosis, Fas receptor is also able to induce the production of proinflammatory cytokines, including TNF-α, a key cytokine in Ps. No differences between Ps patients and controls were reported for CD56dimCD16+ and CD56brightCD16− in peripheral blood [80]. Significantly decreased circulating CD3− CD16+ CD56+ NK cells correlated with increased B lymphocytes in Ps patients were also reported [81]. The role of circulating NK cells in Ps pathogenesis remains still unclear and a subject for further investigation.

In our studies, we identified several phenotypic changes in lymphocyte main populations in the IMQ-mice model. We reported decreased percentages of circulating T-CD4+ and B-CD19+ cells, and elevated levels of T-CD8+ and NK1.1+ cells in IMQtreated mice as compared to healthy animals [82]. For NK cells phenotypic characterization we used a large panel of surface markers including activation, maturation, and markers for cytokine receptors. The results showed important differences in IMQtreated mouse NK cell phenotype as compared to controls [83]. Taking into account the recently found relation between gut microbiota and Ps initiation, we have demonstrated that oral ingestion of IgY raised against pathological gut bacteria resistant to antibiotics can alleviate psoriatic lesions and restore immune parameters [18].

In Ps innate immune cells have been less studied in the peripheral circulation compared to lymphocytes. Besides KCs and T cells, neutrophils are an important cellular source of IL-17 via NETs formation in psoriatic lesions. NETs are increased in blood samples and were correlated with the severity of Ps [84]. Lambert et al. demonstrated that NETs promote Th17 cells induction from PBMCs in Ps. They also suggested that Th17 cells and neutrophils have a cross-talk because this effector helper subset produces cytokines that promote the development, recruitment, and lifespan of neutrophils [85].

Nguyen et al. observed a high expression of the co-stimulatory molecule CD86 on intermediate monocytes (CD14++CD16+ ) in Ps patients, along with high serum beta defensin-2 levels, these parameters positively correlated with PASI score [86]. Recent studies reported that low levels of monocytic myeloid-derived suppressor cells (Mo-MDSCs) are present in the peripheral blood of Ps patients; these cells secrete different pro-inflammatory cytokines (e.g. matrix metalloproteinases 9 and 1, IL-8) moderately suppressing T-CD8+ cell proliferation. According to Soler et al., the immune system is unable to self-regulate due to the capability of Mo-MDSCs to induce aberrant Tregs cell conversion from naive T effector cells, in presence of proinflammatory molecules [87].

ILCs has a significantly increased frequency in the skin, but Villanova and al. observed in the blood of healthy individuals and Ps patients a large amount of CD3 negative (CD3− ) ILC who produce IL-17A and IL-22 (20% of IL-17 producing cells, respectively 40% of IL-22) [88]. NKp44+ ILC3 group is the most frequent subset of ILCs in the peripheral blood and in the skin of Ps patients as compared to healthy individuals. The frequency of circulating NKp44+ ILC3 could reflect the disease severity and/or the response to anti-TNF treatment (adalimumab), highlighting the role of TNF in NKp44+ ILC3 human differentiation [89].

Depending on the δ chain, γδT cells are classified into Vδ1−Vδ3 subsets. In the peripheral blood are present Vδ2 and Vδ3 groups, with the major subset Vγ9Vδ2 which produce a large range of pro-inflammatory mediators (e.g. IL-17A) and activate KCs in a TNF-α and IFN-γ dependent manner. Laggner et al. ha shown in peripheral blood a distinct subset of pro-inflammatory CLA and CCR6 positive Vγ9Vδ2 T cells, which are rapidly recruited into the injured skin [90]. These cells secrete IL-17A,

IL-22 and activate KCs upon TNF-α, IFN-γ, and IL-23 stimulation [65]. Ps patients present low numbers of Vγ9Vδ2+ T cells in peripheral blood and concomitant high levels in psoriatic lesions, both suggesting the pathogenic role of the Vγ9Vδ2+ T subset [90]. The decreased concentration of circulating Vγ9Vδ2+ T cells is normalized after successful Ps treatment. All the data suggest a redistribution of these subsets of γδ T cells from blood to the skin [65].
