New perspectives on ancient granulomas

A Subset of Type I Conventional Dendritic Cells Controls Cutaneous Bacterial Infections through VEGFα-Mediated Recruitment of Neutrophils

Skin conventional dendritic cells (cDCs) exist as two distinct subsets, cDC1s and cDC2s, which maintain the balance of immunity to pathogens and tolerance to self and microbiota. Here, we examined the roles of dermal cDC1s and cDC2s during bacterial infection, notably Propionibacterium acnes (P. acnes). cDC1s, but not cDC2s, regulated the magnitude of the immune response to P. acnes in the murine dermis by controlling neutrophil recruitment to the inflamed site and survival and function therein. Single-cell mRNA sequencing revealed that this regulation relied on secretion of the cytokine vascular endothelial growth factor α (VEGF-α) by a minor subset of activated EpCAM+CD59+Ly-6D+ cDC1s. Neutrophil recruitment by dermal cDC1s was also observed during S. aureus, bacillus Calmette-Guérin (BCG), or E. coli infection, as well as in a model of bacterial insult in human skin. Thus, skin cDC1s are essential regulators of the innate response in cutaneous immunity and have roles beyond classical antigen presentation.

Cutaneous Granulomatosis: a Comprehensive Review

Cutaneous granulomatosis is a heterogeneous group of diseases, characterized by a skin inflammatory reaction triggered by a wide variety of stimuli, including infections, foreign bodies, malignancy, metabolites, and chemicals. From a pathogenic point of view, they are divided into non-infectious and infectious granulomas. Pathophysiological mechanisms are still poorly understood. Non-infectious granulomatous skin diseases include granuloma annulare, necrobiosis lipoidica, rheumatic nodules, foreign body granulomas, cutaneous sarcoidosis, and interstitial granulomatous dermatitis. Necrobiosis lipoidica is more frequent in diabetic patients. Infectious granulomas of the skin are caused by mycobacteria, in particular Mycobacterium tuberculosis or atypical mycobacteria; parasites, such as Leishmania; or fungi. Pathogenic mechanisms of M. tuberculosis-related granuloma are discussed. From a clinical point of view, it is useful to divide cutaneous granulomatosis into localized and more disseminated forms, although this distinction can be sometimes artificial. Three types of localized granulomatous lesions can be distinguished: palisaded granulomas (granuloma annulare, necrobiosis lipoidica, and rheumatoid nodules), foreign body granulomas, and infectious granulomas, which are generally associated with localized infections. Disseminated cutaneous granulomas can be divided into infectious, in particular tuberculosis, and non-infectious forms, among which sarcoidosis and interstitial granulomatous dermatitis. From a histological point of view, the common denominator is the presence of a granulomatous inflammatory infiltrate in the dermis and/or hypodermis; this infiltrate is mainly composed of macrophages grouped into nodules having a nodular, palisaded or interstitial architecture. Finally, we propose which diagnostic procedure should be performed when facing a patient with a suspected cutaneous granulomatosis.

Infections Caused by Mycobacterium tuberculosis in Recipients of Hematopoietic Stem Cell Transplantation

Mycobacterium tuberculosis (M. tuberculosis) infections are uncommon in recipients of hematopoietic stem cell transplantation. These infections are 10-40 times commoner in recipients of stem cell transplantation than in the general population but they are 10 times less in stem cell transplantation recipients compared to solid organ transplant recipients. The incidence of M. tuberculosis infections in recipients of allogeneic stem cell transplantation ranges between <1 and 16% and varies considerably according to the type of transplant and the geographical location. Approximately 80% of M. tuberculosis infections in stem cell transplant recipients have been reported in patients receiving allografts. Several risk factors predispose to M. tuberculosis infections in recipients of hematopoietic stem cell transplantation and these are related to the underlying medical condition and its treatment, the pre-transplant conditioning therapies in addition to the transplant procedure and its own complications. These infections can develop as early as day 11 and as late as day 3337 post-transplant. The course may become rapidly progressive and the patient may develop life-threatening complications. The diagnosis of M. tuberculosis infections in stem cell transplant recipients is usually made on clinical grounds, cultures obtained from clinical specimens, tissues biopsies in addition to serology and molecular tests. Unfortunately, a definitive diagnosis of M. tuberculosis infections in these patients may occasionally be difficult to be established. However, M. tuberculosis infections in transplant recipients usually respond well to treatment with anti-tuberculosis agents provided the diagnosis is made early. A high index of suspicion should be maintained in recipients of stem cell transplantation living in endemic areas and presenting with compatible clinical and radiological manifestations. High mortality rates are associated with infections caused by multidrug-resistant strains, miliary or disseminated infections, and delayed initiation of therapy. In recipients of hematopoietic stem cell transplantation, isoniazid prophylaxis has specific indications and bacillus Calmette-Guerin vaccination is contraindicated as it may lead to disseminated infection. The finding that M. tuberculosis may maintain long-term intracellular viability in human bone marrow-derived mesenchymal stem cells complicates the development of effective vaccines and strategies to eliminate tuberculosis. However, the introduction of linezolid, cellular immunotherapy, and immunomodulation in addition to autologous mesenchymal stem cell transplantation will ultimately have a positive impact on the overall management of infections caused by M. tuberculosis.