Azithromycin Differentially Alters TCR-Activated Helper T Cell Subset Phenotype and Effector Function

In addition to their antibiotic activities, azithromycin (AZM) exhibits anti-inflammatory effects in various respiratory diseases. One of the potent anti-inflammatory mechanisms is through inhibition of CD4+ helper T (Th) cell effector function. However, their impact on specific Th subset is obscure. Herein, we demonstrate the cellular basis of phenotypic and functional alterations associated with Th subsets following AZM treatment in vitro. Using well-characterized Th subset specific chemokine receptors, we report significant suppression of T cell receptor (TCR)-stimulated hyperactivated CCR4+CXCR3+ (Th0) expansion compared to CCR4-CXCR3+ (Th1-like) and CCR4+CXCR3- (Th2-like) cells. Interestingly, this effect was associated with diminished cell proliferation. Furthermore, AZM significantly inhibited the inflammatory cytokines IFN-γ and IL-4 production, CCR4 and CXCR3 receptor expression, and viability of Th0, Th1-like, and Th2-like subsets. Our findings suggest that AZM differentially affects TCR-activated Th subsets phenotype and function, and CCR4 and CXCR3 downregulation and suppressed Th0 subset expansion could potentially influence their trafficking and differentiation into cytokine-producing effector cells.


INTRODUCTION
Azithromycin (AZM) is a potent macrolide antibiotic used to treat various respiratory diseases. Prophylactic treatment with low-dose azithromycin has shown clinical benefits by reducing exacerbations in COPD (1)(2)(3), asthma (4,5), cystic fibrosis (6), and non-cystic fibrosis bronchiectasis (7,8) and mycobacterial infections (9). In addition to their antimicrobial properties, AZM is known to have numerous anti-inflammatory and immunomodulatory effects. Several studies have supported the anti-inflammatory effects of AZM on structural and immune cells (10). Immunomodulatory effects of AZM have also been reported in a recent study on an animal model where treatment resulted in clearance of Bordetella pertussis infection (11). The majority of earlier studies have focused on innate immune cells, mainly monocytes, in context to pro-inflammatory cytokines including IL-1β, TNF-α, and IL8 (12)(13)(14)(15). Given the contribution of CD4+ helper T cells in various respiratory diseases, efforts have been made to understand the immunomodulatory effects of AZM on these cells. Recent studies have shown suppression of CD4+ T cell effector function following AZM treatment potentially by inhibiting proliferation and inducing cell death (10,16). Mechanistically these effects are believed to be due to inhibition of mammalian target of Rapamycin (mTOR) activity (16), involvement of FasL-Fas pathway, and down-regulation of anti-apoptotic protein Bcl-xL (17).
Naive CD4+ T cell recognition of antigens in vivo and polyclonal TCR stimulation in vitro result in cell expansion, differentiation, and effector cytokine production. This changes the dynamicity of chemokine receptor expression pattern on T helper (Th) cells (18,19). Th1 and Th2 are the most characterized and studied Th subsets (20)(21)(22) and are identified by their ability to produce signature cytokines such as IFNγ (Th1) and IL-4, IL-5, and IL-13 (Th2), respectively. The balance between these cells plays a critical role in disease pathogenesis as well as their outcome. Contribution of Th1 and Th2 cytokines have been implicated in the lung infection and chronic inflammation including allergic asthma. In addition to cytokine production, Th1 and Th2 cells upon TCR stimulation acquire different migratory capacities by expressing differential chemokine receptors (19,23) on both polarized and un-polarized cells. For example, Th1 cells predominantly express CXC chemokine receptor 3 (CXCR3) and CCR5 (24-26) while Th2 are known to express CCR3 (27), CCR4 (24, 25,28), and CCR8 (29). After acquiring chemokine receptors, these effector cells then migrate to the site of inflammation in the peripheral tissues.
The previous studies have shown the immunomodulatory effects of AZM on bulk CD4+ T cells (10,16). Given the heterogeneity of helper T cells, it is worth understanding the impact of AZM on Th subsets to delineate their effector function at cellular level. In this study, using well-characterized Th1 and Th2 subset specific chemokine receptors CCR4 and CXCR3, respectively, we identified CCR4+CXCR3+ (Th0), CCR4-CXCR3+ (Th1-like), CCR4+CXCR3-(Th2-like), and CCR4-CXCR3-(DN) subsets in peripheral blood CD4+ T cells. The subset nomenclature is purely based on previous studies (18,19). In this study we report the cellular basis of phenotypic and functional changes associated with AZM treatment of various Th subsets.

Cell Proliferation Assay
Enriched CD4+ T cells were labeled with 2.5 µM carboxyfluorescein succinimidyl ester (CFSE, Invitrogen, United States) for 8 min at room temperature in dark with intermittent mixing. Cells were washed twice with 10 ml of ice-cold complete RPMI 1640 medium and stimulated with immobilized anti-CD3 and soluble anti-CD28 antibodies in presence or absence of azithromycin (Sigma-Aldrich). Cells were harvested on day 3, washed with phosphate buffered saline (PBS, Sigma-Aldrich), and stained with anti-CCR4-AlexaFluor 647 (clone L291 H4, Biolegend, United States) and anti-CXCR3-PECy7 (clone CEW330, eBiosciences, United States) mAbs in FACS staining buffer for 25 min at 4 • C. Washed cells were acquired with flow cytometer (BD FACS Aria III, BD Biosciences, United States) using BD FACS Diva software. Single stain cells were used for flow compensation.

Apoptosis Assay
CD4+ T cells were stimulated with anti-CD3 and CD28 in presence or absence of indicated dose of AZM. Cells were harvested on day 3 and stained with anti-CCR4-AlexaFluor 647 and anti-CXCR3-PECy7 mAbs. Staining was performed with PE-Annexin-V and 7-AAD Detection kit (Biolegend, United States) according to the manufacturer's instructions. Apoptosis was detected with flow cytometer. In some experiments FACS-sorted Th0, Th1-like, and Th2-like cells were also tested for apoptosis using the procedure described above.

Intracellular Cytokine Detection
Anti-CD3 and anti-CD28 stimulated cells were cultured in presence and absence of AZM for 3 days. Brefeldin A, 10 µg/ml (eBiosciences, United States) was added in the last 5 h of the culture. Cells were harvested and surface stained with anti-CCR4-AlexaFluor 647 and anti-CXCR3-PECy7 monoclonal antibodies followed by intracellular Anti-CD3/CD28 stimulated and unstimulated (US) cells were double-stained with anti-IFN-γ and anti-IL-4 mAbs as described in the section "Materials and Methods." FACS plots show the percent cytokine production by AZM treated and untreated cells. Cells without anti-CD3 and CD28 stimulation were taken as control (US). Aligned dot plots show mean ± SEM of total IFN-γ+ (F) and IL-4+ cells (G). Data shown are from three healthy individuals. *P < 0.05, **P < 0.01, ***P < 0.001, ns stands for non-significant.

Sorted Th Subset IFN-γ and IL-4 Cytokine Detection by ELISA
Around 3-5 × 10 4 FACS sorted Th subsets were stimulated as described above with plate-bound anti-CD3 and soluble anti-CD28 in a 48-well culture plate in presence or absence of indicated concentration of AZM. Culture supernatants were harvested on day 3 and stored at −20 • C until use. ELISAs were performed using human DuoSet IFNγ and IL-4 ELISA kits (R&D Biosystems) according to manufacturer's instructions.

Transwell Chemotaxis Assay
Chemotaxis was performed using a 24-well, Transwell plate (8.0µm pore size; Corning, Corning, NY, United States) as described previously (30). Briefly, TCR-stimulated CD4+ T cells culture in presence or absence of AZM were harvested on day 3. Cells were washed with serum-free RPMI 1640 medium and adjusted to a density of 5 × 10 5 cells/ml in serum free RPMI 1640 medium. Medium alone or supplemented with indicated concentration of CCL22 or CXCL11 chemokines was placed in the lower chamber of the Transwell, and 100 ul cells suspension was loaded onto upper chamber. Culture plate was incubated for 120 min in a 5% CO 2 -humidified incubator at 37 • C. Numbers of cells migrating to the lower chamber were counted under a microscope using a hemocytometer.

Ethics Statement
This study was approved by Ethics Committee of Dubai Health Authority (DHA) and Dubai Scientific Research Ethics Committee (DSREC-03/2019_10) United Arab Emirates. Written informed consents were obtained from each participants and study was conducted according to Helsinki Declaration.

Statistical Analysis
For statistical evaluation of data, we have used GraphPad PRISM (v.5.0) software. One-way analysis of variance (ANOVA) followed by Tukey's multiple comparison test to compare more than two groups. For chemotaxis assay unpaired t-test was used for analyzing two groups, and p-value of <0.05 were considered significant.

AZM Suppresses Expansion of TCR-Induced Th Subsets
TCR stimulation is known to trigger CD4+ T cells cell activation, expansion, and cytokine production. At first instance, we attempted to understand the effect of AZM on bulk CD4+ T cells. Enriched CD4+ T cells with purity of above 95% (Supplementary Figure S1A) were stimulated with anti-CD3 and CD28 in presence or absence of various doses of AZM for 3 days. CFSE-labeled cells were cultured in the above condition and proliferation was detected by flow cytometer. We observed significant inhibition in cell proliferation (Figures 1A,D) with greater effect of AZM at 20 µg/ml (P < 0.001). Interestingly, in absence of AZM, the proliferative cells undergo three rounds of cell division on day 3, and that progressively reduced to either two or one division with increasing doses of AZM. To understand potential mechanism of proliferation inhibition, we performed cell death assay on TCR stimulated cells. AZM treated cells undergo significant apoptosis only at higher dose (P < 0.05) as measured by Annexin V labeling (Figures 1B,E).
To determine the CD4+ T cell effector function, intracellular cytokine staining was performed on day 3 of culture in presence and absence of 20 µg/ml AZM. A massive reduction in IFNγ (P < 0.001) and IL-4 (P < 0.001) production was observed following AZM treatment (Figures 1C,F,G). Above observations in cell proliferation, cytokine production and apoptosis are in line with previous studies (10,16).
We then tested if AZM effects we observed on bulk CD4+ T cells differing at Th subset level. Enriched CD4+ T cells were stimulated under similar condition as mentioned above for 3 days. Cells were stained with chemokine receptor-specific anti-CCR4 and anti-CXCR3 mAbs and their frequencies  Figure S1B). However, TCR stimulation resulted in expansion of Th0 subset. Of note, this expansion was drastically reduced by AZM in a dose dependent manner (Figures 2A,B). Nearly four-fold reduction in Th0 subset (P < 0.001) was found at a dose of 20 µg/ml compared to untreated cells. In contrast, DN subset showed a significant increase in cell proportion (P < 0.01).
We also tested the influence of AZM on CCR4 and CXCR3 receptor expression on a per cell basis. AZM treatment resulted in significant decline (P < 0.001) in the mean fluorescence intensity of both receptors in a dose dependent manner (Figures 3A,B). The downregulation of CCR4 is in agreement with previous reports where roxithromycin down modulated CCR4 expression on Th2 cells (31). These data suggest that AZM not only suppresses the cell expansion but chemokine receptor density as well. To understand the functional relevance of CCR4 and CXCR3 downregulation, chemotaxis assays were performed using Transwell system. AZM treated CD4+ T cells showed a general trend of reduced migration to CCR4 and CXCR3 ligands CCL22 and CXCL11, respectively (Supplementary Figures S2A,B).

AZM Suppresses Proliferation of Th Subsets
Next, we sought to determine the effects of AZM on Th subset proliferation. CFSE-labeled CD4+ T cells were stimulated with anti-CD3 and anti-CD28 in the presence or absence of various doses of AZM as described in the section "Materials and Methods." We observed differential proliferative capacities of Th subsets with TCR stimulation as detected by CFSE dilution (Figure 4A). Of note, Th0 subset exhibited highest proliferation compared to others. Interestingly, when we examined the effect of AZM, only Th0 subset at a dose of 20 µg/ml showed a significant proliferative reduction (P < 0.01) compared to other subsets ( Figure 4B). This suggests that TCR-stimulated hyperactivated Th0 subsets are more inclined to AZM mediated proliferative inhibition compared to the rest of the subsets.

AZM Inhibits IFN-γ and IL-4 Cytokine Production by Th Subsets
To determine the effects of AZM on Th subset effector function, we tested Th1-and Th2-associated inflammatory cytokine IFNγ and IL-4 production. Given the significant effects of AZM on Th subset proliferation and apoptosis at 20 µg/ml, we performed intracellular cytokine staining (ICS) assays at same dose. TCR stimulated and unstimulated cells were stained with anti-IFN-γ and anti-IL-4 monoclonal antibodies and detected subsequently with flow cytometer. Various Th subset gated populations were examined for IFN-γ and IL-4 production as done for bulk CD4+ T cells (Figure 1C). TCR stimulation resulted in differential IFN-γ and IL-4 production by various Th subsets compared to unstimulated cells (Figures 5A,B). However, when we tested the influence of AZM, both IFNγ and IL-4 expression were significantly reduced in all except DN subsets. Notably, the magnitude of IL-4 inhibition was much higher (P < 0.001) than IFN-γ (P < 0.01) suggesting a greater influence of AZM on Th2 cytokines. In addition, as control PMA/ionomycin stimulated cells also showed cytokine expression by various Th subsets (Supplementary Figure S5). Furthermore, to understand the effects of AZM on specific Th subsets, FACS-sorted (Supplementary Figure S3) and TCRstimulated Th0 (CCR4+CXCR3+), Th1-like (CCR4-CXCR3 +), and Th2-like CCR4+CXCR3-) cells were cultured in presence or absence of AZM. ELISA results of culture supernatant obtained from two donors show severe inhibition of both IFN-γ and IL-4 cytokine production following AZM treatment (Supplementary Figures S4A,B). These data suggest that AZM not only inhibits cytokine production of bulk CD4+ T cells but also their purified subsets.

AZM Promotes Apoptosis of Th Subsets
One of the mechanisms of AZM mediated inhibition of bulk CD4+ T cell effector function is through apoptotic induction (17). Similar effects were observed in our experiments on bulk CD4+ T cells (Figures 1B,E). To evaluate the apoptosisinducing effect of AZM on various Th subsets, anti-CD3 and CD28 stimulated CD4+ T cells were cultured in presence or absence of AZM for 3 days. Cells were harvested and labeled with viability dye Annexin V and 7-AAD to estimate the level of cell death. Th subsets gated cells were looked at for FIGURE 5 | AZM inhibits IFN-γ and IL-4 production of helper T cell subsets. Anti-CD3 and CD28 stimulated cells were cultured at indicated concentration of AZM for 3-days. BFA was added in last 4 hr of the culture. Cells without anti-CD3/CD28 stimulation were taken as control (US). Intracellular cytokine staining was performed with anti-IFN-γ and anti-IL-4 mAbs and detected by flow cytometer. Th subset cytokine production was determined by gating on specific Th subsets. Aligned dot plots show mean ± SEM of total intracellular IFN-γ+ (A) and IL-4+ (B) production by each subset. Data presented are from four independent experiments performed on healthy individuals (n = 4). *P < 0.05, **P < 0.01, ***P < 0.001, ns stands for non-significant.

DISCUSSION
Given the heterogeneity and chemokine receptor dynamicity within Th cells, we sought to delineate the impact of AZM at the subset level. Herein, using well-characterized Th1 and Th2 subset specific chemokine receptor CXCR3 and CCR4, we show suppressed cell expansion, diminished proliferation associated with enhanced apoptosis, downregulation of CCR4 and CXCR3 expression, and inhibition of inflammatory cytokines IFN-γ and IL-4 by Th cells. These findings suggest the differential response of Th subsets toward AZM. We believe that AZM mediated Th subset alterations could potentially modulate Th0 cell differentiation into Th1 and Th2 cytokine-producing effector cells during inflammatory reactions.
At first instance, we investigated the effect of AZM on bulk CD4+ T cell effector function. We observed proliferative inhibition, diminished IFN-γ and IL-4 cytokine production, and apoptotic induction upon treatment with AZM. These findings are in line with previous studies on CD4+ T cells (10,16). We also observed that in absence of AZM, the proliferative cells undergo three rounds of cell division on day 3 and that progressively reduced to either two or one division with increasing doses of AZM. These data clearly indicate that AZM inhibits cell proliferation by modulating cell cycle. In contrast to other studies where higher doses of AZM were used to induce apoptosis (10,16), our results show significant CD4+ T cells apoptosis even at relatively lower dose (20 µg/ml). Polyclonal TCR stimulation resulted in selective Th0 subset expansion associated with declined proportions of Th1-like, Th2-like, and DN subsets as well as elevated levels of CCR4 and CXCR3 expression on CD4+ T cells. These observations support the notion that TCR stimulation up-regulate CCR4 and CXCR3 expression on activated CD4+ T lymphocytes (18,19,24). However, the elevated levels of CCR4 and CXCR3 expression were significantly downregulated with subsequent treatment of AZM. To understand the functional relevance of AZM mediated CCR4 and CXCR3 inhibition, our chemotaxis assays demonstrated reduced migration of CD4+ T cells in response to their cognate ligands CCL22 and CXCL11. That means AZM could confer anti-inflammatory activity potentially via inhibition of CCR4 and CXCR3 expression, thereby limiting the infiltration of Th subsets to the site of inflammation.
We have observed differential inflammatory cytokine IFNγ and IL-4 production by various Th subsets upon TCR stimulation. Although AZM inhibited both IFN-γ and IL-4 cytokine production, the magnitude of IL-4 inhibition in Th0 and Th2-like subset was higher than IFN-γ, suggesting that AZM greatly influences Th2 cytokine IL-4 production, despite its global suppressive effect on Th cytokines. Moreover, significant production of IFN-γ by Th2-like subsets in our experiments perhaps could be due to an incomplete polarization to Th2 effectors as cells were stimulated only for short duration, and in absence of polarizing cytokines. This observation can also be explained by previous studies where TCR-stimulated Th2like subset shown to acquire higher CCR4 receptor without polarization (18,25,32). The cytokine inhibition capacity of AZM was not limited to bulk CD4+ T cell only as FACS-sorted Th0, Th1-like, and Th2-like cells show massive inhibition.
One of the potential mechanisms of AZM mediated CD4+ T cell suppression is through apoptotic induction (10,16,17). We also observed similar effects when we tested AZM against TCR stimulated bulk CD4+ T cells. In the same vein, Th subsets also exhibited AZM mediated apoptosis of Th0, Th1-like, and DN subsets. While Th2-like subset showed apoptotic resistance, a feature described by others (33)(34)(35). In addition, we also found AZM to significantly reduce the viability of sorted Th1-like, Th2-like, and Th0 subset. Ratzinger et al. has shown a direct effect of AZM on inhibition of mTOR activity as one of the mechanism of CD4+ T cell death (16). Therefore, further investigations are required to understand the involvement of AZM induced mTOR inhibition on Th subsets and other pathways controlling the cell survival.
In this study, our aim was to use bulk CD4+ T cells in order to retain physiologically relevant heterogeneous population and identify Th subsets based on chemokine receptors CCR4 and CXCR3. Although these receptors are well characterized to define Th1 and Th2 subsets (18,19,24), their regulation following TCR stimulation, as observed by others (19,23) as well as our results, may affect the observed phenotype and data interpretation, in particular, the Th expansion results. Perhaps, cytokine driven in vitro polarized Th subsets may provide a better model to understand the cell intrinsic AZM effects on these subsets.
Given the contribution of IL-4 in Th2 cell polarization and IgE production, it is possible that beside AZM inhibition of IL-4 production of Th2, Th0 suppression in early stage may limit the polarization capacity of these cells into Th2 effectors. This could be one of the beneficial anti-inflammatory effects attributed to AZM in Th2 dominated airway diseases such as allergy and asthma. The other possibility could be inhibition of inflammatory cytokine producing Th0 subsets infiltration to the site of inflammation as AZM found to reduce the surface CCR4 and CXCR3 expression on these cells. In summary, our findings suggest that AZM exerts differential immunomodulatory effects on Th subsets. Further studies warrant understanding the precise mechanisms of AZM influence on Th subset differentiation into effectors.

DATA AVAILABILITY STATEMENT
The raw data supporting the conclusions of this article will be made available by the authors, without undue reservation.

ETHICS STATEMENT
The studies involving human participants were reviewed and approved by the Ethics Committee of Dubai Health Authority (DHA) and Dubai Scientific Research Ethics Committee (DSREC-03/2019_10) United Arab Emirates. The participants provided their written informed consent to participate in this study.

AUTHOR CONTRIBUTIONS
AA, FS-A, RAH, and QH conceived and designed the experiments. AA, MJ, and AM carried out the experiments. AA, NS-A, and RH performed the data analysis. AA, FS-A, RH, and RAH wrote the manuscript. AM and TV contributed reagents and analysis tools. QH, BM, and RS provided critical inputs to the manuscript. All authors contributed to manuscript revision, read, and approved the submitted version.

ACKNOWLEDGMENTS
We are indebted to healthy volunteers who participated in this study. We thank members of the asthma research group and Hema Unnikannan for helping with blood collection.

SUPPLEMENTARY MATERIAL
The Supplementary Material for this article can be found online at: https://www.frontiersin.org/articles/10.3389/fimmu. 2020.556579/full#supplementary-material FIGURE S1 | CD4+ T cell purity. (A) FACS plot showing CD4+ T cell purity before (PBMC, left panel) and after (purified CD4+ T cell, right panel) sorting using EasySep separation kit. Cells were stained with anti-CD3 and anti-CD4 antibodies and purity was estimated by flow cytometer. The numbers in quadrant CD3+CD4+ population represent the percent CD4+ T cell purity. (B) Gating strategy to characterize helper T cell subsets. Day-3 anti-CD3/CD28 stimulated or unstimulated purified CD4+ T cells were gated on live cells based on FSC and SSC followed by exclusion of doublets. Singlets were gated to characterize CCR4 and CXCR3 based Th subsets. Contour FACS plot shows four distinct sub-population assigned as CCR4+CXCR3-(Th2-like), CCR4+CXCR3+ (Th0), CCR4-CXCR3+ (Th1-like), and CCR4-CXCR3-(double negative, DN) cells. Th subsets were further gated to determine cell proliferation, viability, and cytokine production in respective experiments.  FIGURE S4 | AZM inhibits cytokine production of FACS sorted Th subsets. Freshly isolated around 3-5 × 10 4 FACS sorted Th subsets were stimulated as described previously with plate-bound anti-CD3 and soluble anti-CD28 in presence or absence of indicated concentration of AZM. Culture supernatants were harvested on day 3 and ELISA was performed. Bar graph show the concentration of IFN-γ (A) and IL-4 (B). Data presented are from two independent experiments from two donors. ND denotes non-detectable.