Altered hematopoietic system and self-tolerance in Bardet-Biedl Syndrome

Bardet-Biedl Syndrome (BBS) is a pleiotropic genetic disease caused by dysfunction of primary cilia. The immune system of patients with BBS or another ciliopathy has not been investigated, most likely because hematopoietic cells do not form cilia. However, there are multiple indications that the impairment of the processes typically associated with cilia might influence the hematopoietic compartment and immunity. In this study, we analyzed clinical data of BBS patients as well as a corresponding mouse model of BBS4 deficiency. We uncovered that BBS patients have higher incidence of certain autoimmune diseases. BBS patients and animal models have elevated white blood cell levels and altered red blood cell and platelet compartments. Moreover, we observed that BBS4 deficiency alters the development and homeostasis of B cells in mice. Some of the hematopoietic system alterations were caused by the BBS-induced obesity. Overall, our study reveals a connection between a ciliopathy and the alterations of the immune system and the hematopoietic compartment.


Introduction
Bardet-Biedl Syndrome (BBS) is a recessive genetic disorder caused by complete or partial loss-of-function mutations in any of more than 20 BBS genes known to date. BBS belongs to a group of ciliopathies, i.e., disorders caused by defective formation and/or function of primary cilia. Eight of the BBS proteins (BBS1, BBS2, BBS4, BBS5, BBS7, BBS8, BBS9, and BBS18) form a transport complex called the BBSome, which sorts selected cargoes into and out of the cilium [1][2][3][4]. Other commonly mutated BBS genes (ARL6/BBS3, MKKS/BBS6, BBS10, and BBS12) assist the BBSome assembly or function [1,5]. The BBSome is believed to act as a cargo adaptor connecting the cargoes to the intraflagellar transport (IFT) machinery [6,7].
BBS is a pleiotropic disease with rod-cone dystrophy, polydactyly, obesity, learning difficulties, hypogonadism, and renal anomalies being the primary diagnostic features [8]. The immune system of patients with ciliopathies including BBS has not been studied in detail. An exceptional study in this respect is a case report of 3 BBS patients suffering from autoimmune diseases in a cohort of 15 studied BBS patients [9]. Similarly, the immune system has not been thoroughly investigated in animal models of ciliopathies either. The possible connection between ciliopathies and the immune system has not been addressed most likely because immune cells do not form primary cilia [10,11]. However, there are several lines of evidence suggesting that the BBS might affect the function of the immune system.
First, the immunological synapse formed between T cells and antigen-presenting cells exhibits a striking analogy to the primary cilium [12,13]. Formation of both the immunological synapse and the cilium involves the reorganization of cortical actin and the centrosome polarization. Along this line, some components of the IFT machinery have been shown to participate in the organization of the immunological synapse to promote T-cell activation [14,15]. In particular, it has been shown that the vesicles containing key T-cell signaling molecules TCR/CD3 complex and LAT are transported towards the immunological synapse by IFT proteins [16,17].
Second, the BBSome is required for Sonic hedgehog (SHH) signaling [18][19][20]. The SHH signaling pathway regulates multiple processes in the organism including T-cell development [21]. In the thymus, SHH regulates the development of thymocytes before and soon after the pre-TCR signaling [22][23][24]. In the periphery, SHH has been shown to negatively regulate TCR-dependent differentiation of T cells [25], as well as to promote Th2 differentiation and allergic reactions [26]. Key components of the SHH signaling pathway, SMO, IHH, GLI1, and PTCH2 are upregulated in effector cytotoxic T cells and transported towards the immunological synapse in vesicles [27]. Moreover, Smo KO/KO T cells showed reduced cytotoxity associated with defects in actin remodeling required for the centrosome polarization and the release of cytotoxic granules [27].
Third, the BBSome regulates trafficking of the leptin receptor [28]. Leptin is a signaling molecule which acts as a pro-inflammatory cytokine [29,30]. In particular, leptin signaling inhibits the proliferation of regulatory T cells [31] and promotes the effector cell proliferation and polarization towards Th1 helper T cells [32]. Moreover, T cells deficient in the leptin receptor show impaired differentiation into Th17 helper T cells in mice [33], indicating a T-cell intrinsic role of leptin signaling.
Fourth, one of the major symptoms of BBS is obesity, which is believed to undermine the immune tolerance [34]. Obesity induces production of pro-inflammatory cytokines, such as TNF-α [35] and IL-6 [36], which might predispose the individual for the development of autoimmune diseases [37][38][39][40][41]. Thus, the BBSome might have an extrinsic role in the immune system via inducing obesity.
In this study, we addressed the intrinsic and extrinsic roles of the BBSome in the immune system by investigating BBS patients and a BBS mouse model of the BBS4 deficiency. We uncovered that BBS patients show elevated prevalence of particular autoimmune diseases. We identified dysregulated homeostasis of blood cells both in BBS patients and in BBS4-deficient mice. Besides, we revealed the association of the BBS induced obesity with specific hematopoietic system alterations in BBS patients.
Bbs4 +/+ and Bbs4 GT/GT or Bbs4 KO/KO littermates were generated by intercrossing heterozygous animals. Mice were bred in specific-pathogen-free facility (Institute of Molecular Genetics) [48]. Animal protocols were approved by the Czech Academy of Sciences, in accordance with the laws of the Czech Republic.
Males and females were used for the experiments. If possible, age-and sex-matched pairs of animals were used in the experimental groups. If possible, littermates were equally divided into the experimental groups.
No randomization was performed since the experimental groups were based solely on the genotype of the mice. The experiments were not blinded since no subjective scoring method was used.

qPCR of BBS genes in immune organs and T cells
Total RNA (1 or 2 μg) of organs (kidney, brain, lymph nodes, spleen) and T cells from C57BL/6J WT mice was obtained in 3 independent biological replicates and transcribed using RevertAid reverse transcriptase (Thermofisher, #EP0442) with oligo(dT)18 primers according to the manufacturer's instructions. RTquantitative PCR was carried out using LightCycler 480 SYBR green I master chemistry (Roche). All samples were measured in triplicates. Obtained CT values were normalized to data of reference genes

Immunoblotting
Freshly isolated murine organs (testicles, thymi, brains) or enriched lymphocytes were homogenized in Laemmli sample buffer. The resulting lysates were separated on a polyacrylamide gel and transferred to nitrocellulose membrane using standard immunoblotting protocols. Membranes were probed with antibodies against BBS4 followed by secondary α-rabbit-HRP antibody. As a loading control we probed the membranes for β-actin followed by secondary α-mouse-HRP antibody. The images were obtained using chemiluminescence immunoblot imaging system Azure c300 (Azure Biosystems, Inc.).

Histological analysis
Testes isolated from 30-day-old male mice were collected, immediately dipped into Bouin solution and fixed for 24 h at 4°C. Paraffin-embedded tissue blocks were cut with a microtome (Leica RM2255), and the sections were stained with hematoxylin/eosin using standard technique. The images were taken using microscope system Axioplan 2 imaging (Zeiss) using 10x/0.50 NA objective.

Weighting of mice
Bodyweight of Bbs4 +/+ , Bbs4 GT/GT , Bbs4 KO/KO mice was recorded weekly starting at 5 weeks of age. All the mice were kept in sex-matched cages together with their littermates (≤ 6 per cage), and fed a standard chow diet ad libitum.

ELISA
Blood from Bbs4 KO/KO , Bbs4 GT/GT and their age/sex-matched controls was collected by submandibular bleeding [49] into EDTA-coated tubes and centrifuged for 15 minutes at 1000 × g at 4°C in order to separate plasma. Obtained plasma samples were assayed immediately or stored at -80 o C for later use. Leptin concentration was measured by mouse leptin ELISA Kit (Cloud-Clone Corp., SEA084Mu) according to the manufacturer's instructions.

Flow cytometry
Live cells were stained with relevant antibodies on ice. LIVE/DEAD near-IR dye or Hoechst 33258 were used for discrimination of live and dead cells. Flow cytometry was carried out using an LSRII (BD Bioscience). Data were analyzed using FlowJo software (TreeStar).

T-cell conjugation assay
T-cell conjugation assay was performed as previously shown [44]. Briefly, OT-I T cells from Bbs4 FL/FL Cd4-Creor Bbs4 FL/FL Cd4-Cre + (cKO) littermates were stained with CFSE cell tracker dye, and splenocytes isolated from C57Bl/6 mice were stained with DDAO cell tracker dye. Splenocytes were loaded with OVA peptide or with indicated altered peptide ligands for 3 h in RPMI/10% FCS, mixed with OT-I T cells at 2:1 ratio, and centrifuged (1000 × g, 1 min). After 20 min of co-culture at 37°C/CO2 incubator, cells were fixed by adding formaldehyde (2% final, 35 min). Cells were centrifuged (1000 × g, 2 min), resuspended in PBS/0.5% gelatin, and analyzed by flow cytometry. Each of the four experiments was carried out in technical duplicates.

Model of autoimmune diabetes
The model of autoimmune diabetes has been described previously [51]. Briefly, OT-I cells from Bbs4 FL/FL Cd4-Creor Bbs4 FL/FL Cd4-Cre + (cKO) sex-matched littermates were adoptively transferred into a host RIP.OVA mice intravenously. On the following day, the host mice were immunized with 5000 CFU of OVA expressing Listeria monocytogenes (Lm). Lm strain expressing OVA has been described previously [52]. Level of glucose in the urine of RIP.OVA mice was monitored on a daily basis using test strips (GLUKOPHAN, Erba Lachema).
The animal was considered to suffer from diabetes when the concentration of glucose in the urine reached ≥ 1000 mg/dl for 2 consecutive days. On day 7 post-infection, blood glucose was measured using contour blood glucose meter (Bayer).

Analysis of the clinical data of BBS patients
Fully anonymized medical records of 255 BBS patients were obtained from the Clinical Registry Investigating BBS (CRIBBS) by the NIH through the National Center for Advancing Translational Sciences and the Office of Rare Diseases Research (https://grdr.hms.harvard.edu/transmart). Data about the prevalence of autoimmune diseases in the CRIBBS cohort were compared to normal prevalence of autoimmune diseases reported in the Autoimmune Registry [53].
Medical records of BBS patients attending the BBS multidisciplinary clinic at Guy's Hospital of Guy's and St Thomas' NHS Foundation Trust, London, or Great Ormond Street Hospital, London, were studied in detail with focus on presence of any immune-related phenotype. In addition to the manual control, the records were also automatically searched for the occurrence of the following terms: autoimm-, immun-, thyro-, inflam-, diabet-, T1DM, ulcerative, crohn, IBD, rheuma-, arthri-, joints. Statistical significance of the difference in the prevalence between the BBS patients and overall population was tested using twotailed binomial test in RStudio (function binom.test).
Results of blood tests of BBS patients (total white blood cell count, leukocyte populations, hemoglobin, platelet counts, mean corpuscular volume, red blood cell count, hematocrit, red cell distribution width and mean corpuscular hemoglobin), their age ranges and body mass indices (BMI) were retrospectively ascertained from medical records stored at the BBS multidisciplinary clinic at Guy's Hospital of Guy's and St Thomas' NHS Foundation Trust, London, or Great Ormond Street Hospital, London. Blood tests were performed during regular medical examination of the patients. All patients gave informed consent or assent.
The protocol for this study was approved by the Great Ormond Street Hospital Research Ethics Committee (Project Molecular Genetics of Human Birth Defectsmapping and gene identification, reference #08/H0713/82) the and by the ethical committee of the Institute of Molecular Genetics of the ASCR.
Two distinct sets of controls for the analyzed set of BBS patients were selected from the 14750 participants of the UK Biobank project (ID: 40103) [54]. First, we selected 10 controls for each patient matching by age range (categories 41-50, 51-60, 60+ years) and sex. These controls had random BMI and thus were used as BMI-random controls. Second, we selected 10 controls for each patient matching by age range (categories 41-50, 51-60, 60+ years), sex, and BMI. These were used as BMI-matched controls. For 34 of the 42 patients we found controls with BMI difference ≤ 0.6 kg/m 2 . For the 8 patients with extreme BMI values, that precise matching was not possible, so that we selected the best-matching controls available for these cases. As the UK Biobank only includes participants older than 40 years, our analysis was limited to this age group.

Enrichment of T and B lymphocytes
T and B lymphocytes were enriched by positive selection using the Dynabeads Biotin Binder kit (Invitrogen, #11047), and biotinylated α-TCRβ and α-CD19 antibodies, respectively.

Autoimmune diseases are more prevalent in BBS patients
In this work, we studied the potential role of the BBSome in the immune system. Initially, we analyzed two cohorts of BBS patients from the CRIBBS NIH registry and from the Guy's Hospital of Guy's and St Thomas' NHS Foundation Trust, London, or Great Ormond Street Hospital, London. We found out that certain autoimmune and inflammatory diseases, such as type I diabetes, Hashimoto's thyroiditis, rheumatoid arthritis, and inflammatory bowel diseases, are more prevalent in BBS patients than in the overall population (Table I). These findings suggested that the BBSome has an intrinsic or extrinsic role in the immune system, particularly in the immune tolerance.
In the next step, we addressed the connection between the BBSome and the immune and hematopoietic systems using mouse models. First, we tested if the BBSome subunits are expressed in the murine immune tissues. We detected the expression of all 8 subunits in the spleen, lymph nodes, and isolated T cells on the mRNA level (Fig. 1A). The expression levels of Bbs2, Bbs4, Bbs9, and Bbs18 in the lymphoid tissues were comparable to, or even slightly higher than in the brain and the kidney, two organs where the BBSome plays a major role [55][56][57][58]. The other four subunits (Bbs1, Bbs5, Bbs7, and Bbs8) were expressed in the lymphoid tissues at 10-to 50-fold lower levels than in the brain and the kidney. Moreover, we detected BBS4 protein in isolated T and B cells (Fig. 1B). Altogether, all the BBSome subunits are variably expressed in lymphocytes and lymphocyte-rich tissue, despite of the fact that hematopoietic cells are commonly considered as non-ciliated cells. This suggested that the BBSome as a whole or some individual BBSome subunits might have an intrinsic role in lymphocytes.

Mouse models for studying the role of the BBSome in the immune system
Our next step was to obtain a mouse model of the BBS. We decided to use the Bbs4-deficient mouse for the following reasons: (I) BBS4 is an essential part of the BBSome [2], (II) Bbs4 KO/KO mouse has been shown to have a relatively severe phenotype in comparison to other BBSome-deficient mice [59,60], (III) Bbs4 had a relatively high expression in lymphoid tissues (Fig. 1A-B). In the following experiments, we used mice with an interrupted Bbs4 gene with a gene-trap (GT) cassette, mice with a deletion of Bbs4 exon 6 (KO), and mice with a Bbs4 exon 6 flanked with LoxP sites for Cre-driven conditional deletion (Fig. 1C).
BBS4 protein was not detectable in the testes and thymi from the gene-trap Bbs4 GT/GT mice (Fig. 1D). As expected, the Bbs4 deficiency lead to the absence of sperm flagella in testes of 30-days-old Bbs4 GT/GT males ( Fig. 1E). However, we were surprised by not observing some previously reported features of BBS mouse models in the Bbs4 GT/GT mice. Whereas mating of Bbs4 +/GT heterozygotes resulted to 17% Bbs4 GT/GT pups at weaning, only 2% Bbs4 KO/KO pups were produced in mating of Bbs4 +/KO heterozygotes (Fig. 1F). This suggests that Bbs4 KO/KO , but not Bbs4 GT/GT , mice suffer from pre-weaning lethality with relatively strong penetrance. Moreover, Bbs4 KO/KO mice, but not Bbs4 GT/GT , developed obesity ( Fig. 1G-I). As expected, adult Bbs4 KO/KO mice, suffering from obesity, had elevated level of leptin in blood plasma in comparison to nonobese Bbs4 GT/GT mice, and pre-obese young Bbs4 KO/KO (Fig. 1J). Because leptin has been proposed to act as a pro-inflammatory signaling molecule, it might have an impact on the immune system of obese BBS mice.
As obesity is caused by the BBSome deficiency in the central nervous system [28], it is plausible that Bbs4 GT allele might retain residual BBS4 expression specifically in the brain, yet we could not detect it (Fig.   1K). Another possibility is that the Bbs4 GT allele allows for an expression of a truncated BBS4 that partially retains its function. In any case, our data suggested that the Bbs4 GT is a hypomorphic allele, which does not lead to a complete loss of the BBS4 function.

Alterations in the immune system of Bbs4 deficient mice
In the next step, we analyzed the development and homeostasis of T and B cells in Bbs4-deficient mice.
We did not observe any major alterations in the T-cell compartment in Bbs4 KO/KO and Bbs4 GT/GT mice ( Fig.   S1A-C). The only significant differences were decreased percentages of CD44 + cells among splenic CD8 + T cells and decreased percentage of T cells among the splenocytes of the Bbs4 KO/KO mice (Fig. S1C).
We observed an alteration of the B-cell development and/or homeostasis in Bbs4-deficient mice. Bbs4 KO/KO , but not Bbs4 GT/GT , mice showed an increased number of B220 + B-lineage cells in the bone marrow.
Moreover, the ratio of B220 high and B220 low cells in the bone marrow was shifted towards less mature B220 low cells in Bbs4 KO/KO and Bbs4 GT/GT mice ( Fig. 2A). Both Bbs4 KO/KO and Bbs4 GT/GT had higher percentage of IgD -IgM -B-cell precursors than controls, although the statistical significance was not reached (Fig. 2B). A deeper analysis of this population showed that Bbs4-deficiency results in a developmental block at the pre-B-cell stage, at which the pre-BCR selection occurs (Fig. 2C).
Interestingly, we observed increased numbers of B cells in the spleen of Bbs4 KO/KO , but not Bbs4 GT/GT mice ( Fig. 2D, Fig. S2A). In the spleen and lymph nodes, Bbs4 KO/KO mice have increased late mature (IgD + IgM -) B-cell population (Fig. 2E, Fig. S2C). Furthermore, Bbs4 KO/KO showed ~2-fold decrease in the percentage of splenic marginal zone B cells (MZB) (Fig. 2F). These phenotypes were largely absent or less pronounced in the Bbs4 GT/GT animals (Fig. 2D, Fig. S2B-C), suggesting that they are caused by obesity and/or caused by the complete loss of function of BBS4 in the full KO animals.
Overall, these data show that BBSome deficiency results in a partial developmental block of B cells in the bone marrow, increase of late mature splenic B cells and relative reduction of splenic MZB cells in the periphery. The developmental block was observed in non-obese Bbs4 GT/GT mice as well, indicating that this phenotype is obesity independent.

The role of Bbs4 in B-cell homeostasis is not intrinsic
To investigate if the observed B-cell developmental block in Bbs4 deficient mice is B-cell intrinsic, we generated Bbs4 FL/FL Vav-iCre mice with a specific deletion of Bbs4 in the hematopoietic lineage.
Surprisingly, we did not observe any signs of the altered B-cell homeostasis in Vav-iCre mice (Fig. 3A-B).
These results suggest that the B-cell compartment in Bbs4 GT/GT and Bbs4 KO/KO mice is affected by factors extrinsic to the hematopoietic lineage, mostly likely the niche in the bone marrow and/or in the peripheral lymphoid organs.

Bbs4 deficiency does not intrinsically influence T-cell and B-cell antigenic responses
As we observed an alteration of the B-cell homeostasis in Bbs4-deficient mice, we decided to investigate how it can affect the response of the adaptive immune system. First, we activated monoclonal B-cells specific to 4-hydroxy-3-nitrophenyl acetyl (NP) from B1-8 [42] and Bbs4 GT/GT B1-8 mice using NP-labeled cells and monitored the upregulation of CD69 activation marker. In this assay, we did not observe any role of Bbs4 deficiency in the B-cell response (Fig. 4A-B). These experiments were only performed in Bbs4 GT/GT animals to exclude the possible role of obesity as an extrinsic factor.
Taking into account the arising evidence that the primary cilium might share some features with the immune synapse in T cells [13,17,61], we expected that the T-cell functions might be compromised in the Bbs4deficient mice. For this reason, we generated a Bbs4 FL/FL Cd4-Cre mouse line where Bbs4 deficiency was restricted to T cells (Fig. S3A). These mice did not show any developmental alterations in the T-cell compartment ( Fig. S3B-E). To study the role of BBSome in the T-cell antigenic responses, we crossed the Bbs4 FL/FL Cd4-Cre to TCR transgenic OT-I Rag2 KO/KO mice. This mouse generates monoclonal OT-I T cells specific for K b -OVA, a model antigen originating from chicken ovalbumin. We did not observe any role of BBS4 in the conjugation of OT-I T cells with antigen presenting cells loaded with OVA or with altered peptide ligands with variable affinity (Fig. 4C). Moreover, WT and Bbs4-deficient OT-I T cells showed the same ability to induce autoimmune diabetes upon a transfer into RIP.OVA mice expressing ovalbumin under the rat insulin promoter and subsequent priming by Listeria monocytogenes expressing ovalbumin [44,52] (Fig. 4D-E). This assay examines OT-I T cells for a complex of abilities, i.e., priming by the OVAantigen, expansion, infiltration of the pancreas, and killing the β-cells. As the onset of diabetes caused by Bbs4-deficient OT-I T cells was not different from the control, we concluded that BBS4 does not play an important intrinsic role in any of indicated steps of the T-cell-mediated immune response.

BBS-induced obesity affects blood homeostasis
To investigate the possible factors predisposing BBS patients to the development of autoimmune diseases, we decided to examine the blood test results of BBS patients. Intriguingly, immunity-related parameters, such as counts of total white blood cells, neutrophils, and eosinophils, were increased in BBS patients, when compared to age and gender matched controls (BMI-random controls) (Fig. 5A). To address the role of obesity in BBS patients, we decided to use an additional set of controls with body mass indexes (BMI) matching to those of BBS patients (BMI-matched controls). We did not observe any difference when we compared the indicated leukocyte parameters between BBS patients and BMI-matched controls. In addition, we analyzed the peripheral blood of Bbs4 deficient mice. In agreement with the analysis of the patients' blood tests, we did not observe major differences between WT and non-obese Bbs4 GT/GT mice.
However, obese Bbs4 KO/KO mice showed higher total white blood cell count than WT controls (Fig. 5B) in line with the data from patients (Fig. 5A). These results indicate that obesity in BBS patients and in Bbs4deficient mice has an impact on the leukocyte homeostasis. Moreover, BBS patients showed significantly higher C-reactive protein (CRP) levels than BMI-matched and BMI-random controls (Fig. 5C), which indicates that obesity is not the only factor influencing the immune system of BBS patients.
Notably, the homeostasis of red blood cells was altered in BBS patients as well as in the Bbs4 KO/KO mice, although at different levels ( Fig. 5D-E). BBS patients showed low overall hemoglobin levels caused by a mild decrease in the red blood cell count and low red blood cell hemoglobin (Fig. 5D), indicating a possibility of a reduced oxygen transport capacity. Interestingly, the comparison of BBS patients with BMImatched controls showed that the alteration of the erythroid compartment was not caused by obesity. The mouse model of BBS showed a decreased number of red blood cells, which was compensated by enlarged red blood cell volume (Fig. 5E).
In addition, we observed decreased platelet counts both in BBS patients and Bbs4 KO/KO mice ( Fig. 5F-G).
The reduction of platelets in BBS patients was not obesity-dependent. Furthermore, Bbs4 KO/KO mice showed higher mean platelet volume and lower platelecrit than WT mice (Fig. 5F-G), indicating enhanced removal of platelets in the periphery.
Altogether, our results suggest the role of the BBSome in the immune tolerance, hematopoiesis and/or blood homeostasis. Most of the effects seem to be extrinsic to the hematopoietic compartment as revealed by using tissue-specific knock-out mouse model and comparison of patients to BMI-matched controls.
However, altered CRP levels, red blood cell, and platelet homeostasis in BBS patients are obesityindependent.

Discussion
In this work, we have shown that the BBS is connected with changes in the immune system and blood homeostasis and with higher incidence of autoimmune diseases. First, Bbs4 depletion affects B-cell development and red blood cell and platelet homeostasis, and second, it leads to obesity, which induces changes in the immune populations.
Lymphocytes are usually considered as a classical example of non-ciliated cells [10,11], although some contradicting studies were published [62,63]. Importantly, T cells repurpose a number of proteins associated with the ciliary transport for the formation of the immunological synapse. Three prominent signaling molecules in T cells (TCR, LAT, LCK) are transported to the immunological synapse via the intraflagellar transport machinery (IFT) [15,17,61]. As the BBSome directly interacts with the IFT [64-66], we hypothesized that the BBSome might have a cilia-independent role in the T-cell activation.
However, our data do not support this hypothesis, because the immune response of Bbs4-deficient T cells was not affected.
Some ciliary proteins, including BBSome-interacting partner IFT20, and SHH signaling components, play a role in the early T-cell development. It has been shown that the Lck-Cre driven ablation of Ift20 in early stages of T-cell development impairs the maturation of thymocytes, leading to the reduction of peripheral T cells [67]. Interestingly, Ift20 ablation at later stages of T-cell development using Cd4-Cre had only mild effect on the T-cell maturation [15,67]. Similar findings were shown using Lck-Cre and Cd4-Cre driven ablation of SMO, a receptor involved in SHH signaling [24,68]. In our study, the T-cell lineage was largely unaffected in the Bbs4 KO/KO mouse and in Cd4-Cre driven Bbs4 cKO/cKO mice, which suggests that the BBSome is dispensable for SHH signaling in thymocytes and for T-cell development in general. Bbs4 KO/KO mice, similarly to the majority of BBS patients, develop obesity which influences the immune system. Obesity can induce the state of low-grade metabolic inflammation [72], characterized by elevated TNFα, IL-6, and CRP in blood [73][74][75][76] and adipose tissue [77]. Obesity is a risk factor for some autoimmune disorders, including multiple sclerosis [78][79][80], systemic lupus erythematosus [81], rheumatoid arthritis [82][83][84][85], and autoimmune diabetes mellitus [86,87]. The risk of Raynaud's and celiac diseases is decreased in obesity [86], and the risk of inflammatory bowel disease [88][89][90] and hypothyroidism [91][92][93] remains controversial. In this work, we have shown elevated incidence of certain autoimmune disorders in BBS cohorts, as well as altered composition of blood cells in BBS patients and BBS mouse models. The comparison of BBS patients and non-BBS obese controls showed that the alteration of the white blood cell count in BBS patients is caused by obesity, which was supported by the murine data. Altogether, our results suggest that obesity might substantially contribute to the high incidence of certain autoimmune diseases in BBS patients. However, the data also indicate that obesity is not the only factor altering the immune status of BBS patients. This is showed for example by the increased CRP level in BBS patients, which can not be fully explained by the high BMI. It should be noted that BBS patients are under various medications, some of which can potentially alter the blood homeostasis.
One of the major players in obesity-associated inflammation is leptin, an adipocyte-derived hormone which acts as a pro-inflammatory cytokine [94,95]. Leptin receptor is expressed by different cell types including T cells, in which it promotes T-cell activation and proliferation. [32,96]. It has been shown that the BBSome is required for the cell surface delivery of the leptin receptor in human kidney epithelial cell line HEK293 [28]. However, we did not observe immune response defects in murine T cells deficient in Bbs4, implying that the BBSome is not required for the proper function of the leptin receptor in T cells.
In addition, we observed thrombocytopenia both in BBS patients and in Bbs4-null mice; and furthermore, platelets from these mice had larger volume. There are two major causes of thrombocytopenia, the first one is a decreased production of platelets in the bone marrow, and the second is an over-destruction of platelets on the periphery [97]. Since the size of the immature platelets is larger than that of the mature ones, large platelet volume combined with thrombocytopenia indicates active platelet production in the bone marrow compensating for their loss in the periphery [98]. This state can be a result of immune-mediated processes such as immune thrombocytopenia; it is also seen in disseminated intravascular coagulopathy, thrombotic thrombocytopenic purpura, and sepsis [99,100]. Unfortunately, we do not have results of mean platelet volume from BBS patients, but we can presume that the reasons of thrombocytopenia in BBS patients and mice are similar, suggesting a role of the peripheral destruction of platelets.
To our knowledge, the immune system in ciliopathies has not been investigated yet, although it can be affected in multiple ways. First, obesity is a common feature of ciliopathies such as BBS and Alström syndrome [101]. Second, the deficiency of ciliary proteins might have different effects on immunity, including immunological synapse defects, which needs to be further elucidated. Third, non-hematopoietic ciliated cells interact with leukocytes and play a role in the immune defense. Here, we show that the absence of a ciliary protein BBS4 affects the immune system of humans and mice in two ways, i.e., obesitydependent (changes in leukocyte homeostasis), and obesity-independent (high CRP levels in BBS patients, partial developmental block in the B-cell lineage).

Acknowledgement
We thank Ladislav Cupak for technical assistance and genotyping of mice and Dr. Alena Moudra for the assistance with Listeria infection. This study was supported by the Czech Science Foundation (17-20613Y to

Conflict of interest
All authors declare that they have no conflict of interest. Tables   Table 1. Autoimmune diseases in BBS patients. The table shows the fold change in the prevalence of autoimmune diseases in the CRIBBS cohort of 255 BBS patients (upper part) and in the cohort of 198 BBS patients from the Great Ormond Street Hospital and the Guy's Hospital in London (lower part). Normal prevalence of autoimmune diseases was adopted from the Autoimmune Registry [53]. P value was calculated using binomial test.      Hospital were extracted from the medical records and compared to two sets of healthy controls obtained from the UK Biobank. The BMI-random controls were age-and gender-matched to the set of BBS patients, with random BMIs. The BMI-matched controls were matched for age, gender, and BMI. Both data sets of healthy controls were selected as 10-fold larger than the set of BBS patients to obtain higher statistical power. Median is shown. Kruskal-Wallis test was used for the statistical analysis in A, D and F. In C, the percentages of patients or controls having CRP≥5 were compared using Fisher's exact test with post-hoc Sidak correction for multiple comparisons. (B, E, G