09/02/2022
AIDS, COLLAGEN, BONE MARROW
Part II.
Patients with AIDS showed significantly reduced trabecular bone thickness (Table 1). A significant positive correlation was observed between the trabecular bone thickness and age of patients with AIDS (rS = 0.0349; P = .006), and a significant negative correlation was observed between the trabecular thickness and BMI in the same patient group (rS = −0.156; P < .001).
The percentage of collagen fibers in the trabecular bone was significantly higher in patients with AIDS (Fig. 1, Table 1). A negative and statistically non-significant correlation was observed between the percentage of collagen fibers and the age of patients with AIDS (rS = −0.0107; P = .600), and a negative and statistically significant correlation was observed between collagen percentage and BMI in the same patient group (rS = −0.024; P < .001).
4. Study limitations
As this study was conducted using data collected from autopsy reports, many relevant information for the bone health it cannot be obtained, such as smoking, menopause, use of hormone therapy, use of antiretroviral therapy, CD4 count and viral load at the time of death, among other factors that possibly influence bone formation. A cross-sectional study conducted in São Paulo (Brazil), which included 4.332 women over 40 years who were treated in primary care service, observed the prevalence of osteoporosis (33%) and osteoporotic fractures (11.5%) in these patients [15]. Hypoestrogenism is considered an important risk factor for low bone mineral density and recent studies correlated the occurrence of osteoporosis with the time of menopause. The same reasoning applies to the use of hormone therapy for prevention of osteoporosis. The beneficial effects of estrogen therapy on the preservation of BMI and reduce the risk of fractures are well established in the literature [16]. A World Health Organization study indicated that smoking confers substantial risk for future fracture, regardless of a BMI [17]. Researchers have shown that individuals with low CD4 levels have low bone mineral density; antiretroviral therapy affects the bone structure as leads to a recovery of bone turnover [18].
5. Discussion
The changes of the bone marrow have been the subject of several studies in patients with HIV/AIDS. Its different constituents, such as trabecular bone, collagen fibers, and cellularity show up in the affected syndrome.
In this study, patients with AIDS had significantly reduced trabecular bone thickness. Another study showed that the prevalence of osteopenia and osteoporosis is 3-fold higher in patients infected with HIV than in non-infected patients. The mechanisms responsible for these changes are still not completely understood. However, chronic inflammation caused by HIV infection was associated with bone resorption, and the virus itself may also directly affect osteoclast activity [19], [20]. HIV Tat, a regulatory protein of viral transcription, has been found to increase differentiation of precursor cells into osteoclast [21]. Tat viral protein was associated with increased osteoclast activity formation by 70%, HIV Tat being biologically active in driving a pro-osteoclastic phenotype [22]. The balance between bone formation and reabsorption becomes progressively negative with aging, and most of bone loss occurs after the age of 65 years, which is more intense in women due to the drop of estrogen levels [23]. However, a significant proportion of trabecular bone loss throughout life is age related and estrogen independent, since trabecular bone is not as sensitive to the estrogen effects in maintaining bone mass as the cortical bone [24], [25].
Some studies indicate that oxidative stress, which increases with aging and is accentuated by s*x steroid deficiency, may be an important factor leading to impaired bone formation with aging [24]. The balance between bone formation and resorption becomes increasingly negative with increasing age, and most of the bone loss occurs after 65 years of age and is actually more intense in women due to the sudden drop in estrogen levels [23]; however, a significant proportion of trabecular bone loss is directly related to age, regardless of estrogen levels; this is because the trabecular bone does not have the same sensitivity to estrogen in terms of bone mass maintenance purposes, as with the cortical bone [24], [25]. Some studies suggest that increased oxidative stress, which occurs with aging and is marked by a deficiency of s*x steroids, may be an important cause of poor bone formation [25].
Collagen deposition in trabecular bones was significantly higher in patients with AIDS and was more pronounced in younger patients and those with lower BMI. Mature osteoblasts synthesize molecules involved in bone formation or regulation of osteoclast activity, such as collagen type I. These osteoblasts may be infected with HIV and directly affected by viral replication [26]. Some studies have shown that low body weight contributes to low bone mineral density, besides increasing the risk of fracture, especially in older women. Increased peripheral conversion of gonadal hormones in obese patients improve the maintenance of bone mass, protecting against the adverse effects of estrogen deficiency on the skeleton [16]. BMI lower than 25 kg/m2 was associated with a 3-fold increase in the risk of developing osteoporosis and the protective effect of body weight or BMI on bone has previously been reported. The conversion of androgens to estrogens is an important source of circulating estrogens; it occurs in adipose tissue and bone cells, and the local concentration of estradiol seems to contribute significantly to skeletal homeostasis. Collagen loss plays a major role in the pathogenesis of osteoporosis. It has been postulated previously that hormone reposition therapy in menopause leads to an increase in bone mass by an increase in the collagen levels in bone, by elevating estrogen levels [27], [28].
Although this study did not observe a significant change in cellularity, a variety of nonspecific morphological abnormalities may be observed microscopically in the bone marrow of patients infected with HIV. Global hypercellularity may occur in the initial course of AIDS or during systemic infections because of cellular hyperplasia of granulocyte lineages and megakaryocytes. Increased hypocellularity and adipose tissue atrophy have been observed as consequences of the debilitation that occurs in later stages of AIDS [29], [30].
Anatomically, the spaces in the bone marrow are compartmentalized, allowing immune and bone cells to interact and influence each other. Therefore, bone homeostasis is often influenced by immune responses, particularly when the immune system is activated or compromised [1]. Increasing evidence indicates that bone marrow suppression is caused by viral infection of accessory cells, which results in impairment of stromal function and an altered hematopoietic growth factor network [31].
In this study, the bone marrow of patients with AIDS showed thinning and increased collagen deposition in trabecular bone. The bone marrow stroma consists of a mixture of cellular and extracellular components, namely fibroblasts, adipocytes, endothelial cells, macrophages, adhesion molecules, and growth factors and, when amended, contributes to the development of hematological abnormalities in patients with HIV/AIDS [32], [33]. Fibroblasts are an important source of trophic factors or inhibitory regulating the development of hematopoietic progenitor cells. Direct infection of stromal components such as macrophages, megakaryocytes, and fibroblasts convert these cells into HIV reservoirs and inhibit their ability to support growth of hematopoietic progenitor cells [32], [33], [34].
Infection of these cells can result in deregulated expression of adhesion molecules and regulatory cytokines and play an important role in the development of cytopenias and susceptibility to infections [35], [36]. Immune dysfunction in HIV-infected individuals are primarily due to high levels inflammatory cytokines such as TNF-α, IFN-α, IFN-γ, and TGF-β, which are present in bone marrow biopsies and serum of patients with the syndrome [37].
Previous experimental studies on HIV/AIDS have shown that collagen production and pathological deposition were associated with early induction of immunoregulatory response in secondary lymphoid tissues consisted of regulatory T cells as well as other types of cells expressing TGF-β [38], suggesting that this cytokine is an important pro-fibrotic signal from HIV/AIDS disease.
Some authors also show a statistical correlation between CD41 lymphocyte count and the degree of reticulin fibrosis, indicating that the mechanism of bone marrow fibrosis is independent of the disease state [39], [40].
In these patients, abnormalities are often observed in all bone marrow cells, including the matrix. It is hoped that this study contribute to further studies that could demonstrate the relationship of bone marrow changes with the immune and nutritional status of patients infected with HIV and the presence of opportunistic infections. The interpretation of the evaluated parameters in this study can be very helpful in the diagnosis of conditions associated with HIV infection, contributing to clinical improvement and life expectancy of the patient.