**1. Introduction**

Rheumatoid arthritis (RA) is an autoimmune condition, characterized by symmetrical joint inflammation, that affects approximately 1% of the world's population [1]. RA is characterized mainly by synovium hyperplasia and a joint destruction process. In this scenario, immune cells and the inflammatory microenvironment that they create in affected joints are key components in the pathophysiology of RA. Moreover, it is well-described that during the inflammatory stages of the disease, extra-articular manifestations are common, which involve other tissues or organs [2].

RA patients are at risk of systemic complications and several co-morbidities, including osteoporosis and frequent vertebral and hip fragility fractures [3,4]. The incidence rate of overall fractures in RA patients is 33 per 1000 person-years [5], and the risk is increased with disease activity and associated with overexpression of pro-inflammatory cytokines that can disturb the bone remodeling process [4,6,7].

Over the last decades, animal models—especially the rodent models—have been crucial tools for understanding the biologic process of RA [8], and their use can aid in developing new therapeutic strategies for fracture healing in RA inflammatory conditions. Collagen induced-arthritis (CIA) animal models have been one of the most widely used models in RA research. Originally described by Trentham [9], CIA is a reproducible animal experimental model of RA [10,11]. In fact, the similarity to human RA regarding the disease clinical, histological, and immunological signals—including high articular levels of inflammatory cytokines—like tumor necrosis factor-α (TNF-α) [12,13], make it an invaluable model to study the pathologic process and to search new therapeutic strategies [14–16]. The response to rat CIA has been reported to involve macrophages, T and B lymphocytes, and mediators such as TNF-α, Interleukin (IL)-1β, IL-6, and IL-17 [17]. Nonetheless, the systemic response in this model has not been well characterized so far.

Importantly, current RA treatments do not promote joint repair, and several efforts are being made to develop new therapies, especially based in cell approaches using mesenchymal stem/stromal cells (MSC) [18,19]. MSC are multipotent progenitor cells with the potential to differentiate into mesenchymal lineage tissues (e.g., bone, cartilage, and adipose tissue), described to have immunomodulatory roles [20], being capable of recruiting different cell types and promoting tissue repair [21]. The transplantation of MSC has been reported to ameliorate or delay RA onset in CIA animals, partially mediated by inflammatory signaling suppression [22,23], and thereby reducing joint swelling and destruction [24,25]. Although the available evidence supports the use of MSC transplantation as a cell-based strategy in CIA animals, data on the biology of endogenous CIA animals-derived MSC in basal conditions is scarce. Moreover, the impact of the systemic inflammatory condition on biological properties of endogenous MSC has not been explored yet.

Herein, we propose CIA as a reliable model to study bone regeneration in inflammatory conditions, and additionally we investigate the effect of RA induction on the biological behavior of endogenous MSC as crucial cells involved in repair/regeneration.

Our results have shown that the combination of the two models is feasible and that CIA animals respond to the bone injury with a significant increase of systemic myeloid cells number and their co-stimulatory molecules (CD40 and CD86) expression, accompanied by increased levels of IL-13, IL-2, and IL-6 in plasma. The systemic inflammatory environment created by the arthritis induction leads to decreased metabolic activity and proliferation of CIA-derived MSC, and increased differentiation capacity determined by the expression of osteogenic (runt-related transcription factor 2 (RUNX2) and alkaline phosphatase (ALP)), and chondrogenic (aggrecan (ACAN)) markers in basal conditions.
