Chemokines in innate and adaptive granuloma formation

Abstract

Granulomas are cellular inflammations that vary widely in histologic appearance depending upon the inciting agent and immunologic status of the responding host. Despite their heterogeneity, granulomas are at their core an ancient innate sequestration response characterized by the accumulation of mononuclear phagocytes. In fact, this innate cellular response was first observed by Metchnikov in simple invertebrates. Among higher vertebrates, environmental pressures have resulted in the evolution of more sophisticated adaptive immune responses which can be superimposed upon and modify the character of granulomatous inflammation. Compared to immune responses that rapidly neutralize and eliminate infectious agents, the granuloma represents a less desirable "fall back" response which still has value to the host but can be co-opted by certain infectious agents and contribute to bystander organ damage. Understanding granulomas requires an analysis of the complex interplay of innate and adaptive molecular signals that govern the focal accumulation and activity of their cellular components. Among these signals, small molecular weight chemoattractant proteins known as chemokines are potentially important contributors as they participate in both directing leukocyte migration and function. This tract will discuss the contribution of chemokines to the development of innate and adaptive granuloma formation, as well as describe their relationship to more recently evolved cytokines generated during adaptive immune responses.

Keywords: chemokine receptors; chemokines; granuloma; inflammation; innate immunity.

Figure 1

Potential fates of the innate granuloma. In the immunocompetent host the ultimate type of granuloma manifested will depend in large part on the nature of the inciting agent. Weakly antigenic agents with poor expression of pathogen (PAMP) and or damage (DAMP) associated molecular patterns will result in a persisting low turnover innate granuloma. Examples would be surgical implant and suture granulomas. Weakly antigenic agents associated with greater PAMPs or DAMPs will activate a broader spectrum of innate responding cells resulting in more complex higher turnover innate granulomas. Examples would be silica, zymosan, and muramyl dipeptide induced granulomas. Strongly antigenic agents with associated PAMP or DAMP signals will initiate additional adaptive antigen-specific T cell responses resulting in a complex high turnover granuloma. The types of cellular components will depend upon the participating T cell subset. Examples would be mycobacterial, fungal, and parasite ova elicited granulomas. It should be noted that humoral elements including antibodies and complement may also influence lesions. Lower panels show examples of the histologic appearance of the various granulomas. (A) Innate low turnover foreign-body granulomatous response to surgical bone cement; (B) Innate high turnover granulomatous response to silica; (C) Adaptive granulomatous response to M. tuberculosis, Th1 type; (D) Adaptive eosinophil-rich, granulomatous response to schistosome parasite egg, Th2 type. Original magnifications 200×.

Figure 2

Synchronized innate and adaptive antigen bead-elicited pulmonary granuloma formation. (A) Histologic appearance and sizes of innate antigen bead granulomas. Agarose beads covalently bound to Mycobacterium bovis purified protein derivative (PPD), Schistosoma mansoni soluble egg antigens (SEA), or bovine serum albumin (BSA), (CON) were administered intravenously to naïve C57BL/6 mice at a dose of 6000 beads per mouse. Lungs were harvested at the designated time points for histologic examination and cross-sectional measurement using computer assisted image analysis. A minimum of 20 lesions were measured. Dashed line represents average bead size. (B) Histologic appearance and sizes of adaptive antigen bead granulomas. Antigen beads were administered as above, but mice were pre-sensitized to the respective mycobacterial and schistosome egg antigens. Lungs were harvested at the designated time points for histologic examination and cross-sectional measurement as above. Size of a non-antigen coated control bead reaction is shown in chart for comparison. Bars are means and standard deviations of five mice.

Figure 3

Cellular composition of innate and adaptive antigen bead-elicited pulmonary granulomas. (A) Time course of major cell populations recruited during adaptive and innate granuloma induction. Innate BSA and adaptive PPD- and SEA-antigen bead lung granulomas were elicited as described in Figure 2. On the indicated days, granulomas were harvested and enzymatically dispersed, washed, and then used for differential analysis. (B) Day 4 cell composition of innate and adaptive PPD- and SEA-antigen bead granulomas. Granulomas were elicited and harvested as above and then washed cellular suspensions were subjected to differential analysis to determine cellular components. Lymphocytes (Lymp), large mononuclear cells (Large mono), eosinophils (Eos), and neutrophils (Neu) were determined by standard morphologic analysis. Flow cytometry was used to identify lymphoid subpopulations CD4+ and CD8+ T cells, natural killer (NK+). Bars are means and standard deviations of five mice.

Figure 4

Dynamics of chemokine transcript expression during innate and adaptive antigen bead-elicited granulomas formation. Innate and adaptive PPD- and SEA-antigen bead granulomas were elicited as described in Figure 2. Lungs were harvested at the designated time points for mRNA extraction and cDNA preparation. Chemokine transcript levels were determined by semiquantitative real-time RT-PCR. Mean arbitrary units (AU) were derived from five to six individual lungs per point.