BLyS drives affinity maturation

I have found an interesting paper that describes in the very detailed way how the interactions between two specialized subsets of lymphocytes which are germinal center (GC) B cells and follicular helper (FH) T cells may influence the affinity maturation of antibodies. The cooperation of these two subsets is known for the long time. However, the novelty this publication brings about is that it looks specifically at the germinal center reaction and dissects the role that the molecule called BLyS (or interchangeably BAFF) may play in the affinity maturation of antibodies from other tasks fulfilled by the same protein. In other words, BLyS has important functions in many aspects of B cells life and the exact definition of its role in the affinity maturation process was not possible using relatively straightforward methods like studying mice with deletion of gene encoding BLyS.

The link:

Authors perform the study on mice that were immunized with nitrophenacethyl hapten conjugated with chicken gamma globulin which is a widely used method to elicit a strong germinal center reaction. What forms the basis of this paper is the observation that receptor-bound BLyS seems to be selectively excluded form B cells that are located inside germinal centers. Investigators follow with the demonstration that activated B cells downregulate TACI (one of three BLyS receptors), that IL-21 may be responsible for the observed TACI downregulation and finally that in the context of germinal center (and unlike systemically) the main source of BLyS comes from cells of hematopoietic origin, namely follicular helper T cells.

But what role the T cell-derived BLyS may have for the quality of the antibody response? To answer this question authors create an experimental setting (mixed bone marrow chimeras) where the T cell lineage lacks BLyS. In such circumstances germinal centers do form and are maintained mostly normally. However, the ability to form high-affinity antibodies is visibly impaired when germinal centers operate without T-cell derived BLyS.

Radhika Goenka, Andrew H. Matthews, Bochao Zhang, Patrick J. O’Neill, Jean L. Scholz, Thi-Sau Migone, Warren J. Leonard, William Stohl, Uri Hershberg, and Michael P. Cancro (2014). Local BLyS production by T follicular cells mediates retention of high affinity B cells during affinity maturation Journal of Experimental Medicine DOI: 10.1084/jem.20130505

IL-21 reporter mouse shines more light on follicular helpers

Follicular helpers are the special population of CD4 T cells that localize to germinal centers and are involved in enhancing the humoral response. They are distinguishable from other CD4 T cell subsets by their joint expression of CXCR5, ICOS, PD-1 and the transcription factor Bcl-6 as well as the capacity to secrete IL-21 cytokine. The recent paper describes a novel model to study the biology of TFH – the IL-21 reporter mouse. Such mouse has a GFP encoding cassette knocked-in into IL-21 locus to create the animal that expresses simultaneously IL-21 and GFP. IL-21 is not a cytokine characteristic exclusively for TFH but in conditions applied by this study the GFP presence in secondary lymphoid organs is restricted to activated CD4 T cells that are positive for CXCR5 and PD-1 thus being follicular helpers according to the accepted definition. The ability to track TFH population in vivo allows answering questions concerning their fate after the primary immune response is over and germinal centers undergo resolution.

The link:

There are two main contributions that this paper adds to the existing knowledge of TFH. First, it dissects follicular helpers into IL-21 secreting and IL-21 negative populations and characterizes both subsets. Second, it analyzes the ability of TFH to form the memory compartment and participate in recall responses. In order to track IL-21 secreting cells among TFH authors follow general follicular helper population (defined as CD4+CD44+CD62LCXCR5+PD-1+cells) after the immunization with NP-KLH (a T cell-dependent antigen). They discover that GFP-positive cells constitute a stable proportion of TFH (~30-40%) at every time points during the course of primary response. Both IL-21 secreting (GFP+) and IL-21 negative (GFP) subpopulations are mostly equivalent in terms of transcription factors (Bcl-6, IRF4, Blimp-1, T-bet and GATA3 tested) and cytokine expression (IFN-γ, IL-4 and IL-10 examined), proliferation and functional abilities (NP-specific GC B cell response analyzed). The only difference between two subsets was an increase in transcripts encoding T-bet, IFN-γ and IL-10 among GFPcells.

The potential of TFH to participate in recall responses is tested by adoptive transfer approach. Follicular helpers generated in the course of immune response to influenza virus (flu infection similarly to NP-KLH immunization leads to the formation of GFP+ and GFP– subsets of TFH) were injected into congenic recipients challenged subsequently with the same virus. It is observed that antigen-experienced  TFH have the ability to expand on re-stimulation in vivo. Transferred follicular helpers are then analyzed by their surface marker expression and cytokine profile.  In secondary lymphoid organs TFH have a definite propensity to continue as TFH. However, at effector sites (lungs) the vast majority of former follicular helpers become conventional effectors CD4 T cells. Follicular helpers retain also the substantial ability to release cytokines on antigen re-exposure. It is interesting to know that TFH have some plasticity and may end up doing different tasks after germinal centers of primary immune response are disbanded.

Lüthje, K., Kallies, A., Shimohakamada, Y., Belz, G., Light, A., Tarlinton, D., & Nutt, S. (2012). The development and fate of follicular helper T cells defined by an IL-21 reporter mouse Nature Immunology, 13 (5), 491-498 DOI: 10.1038/ni.2261

Germinal centers in pathogenic SIV infection

The characteristic feature of HIV and some SIV infections is the chronic immune activation that probably drives the continuous CD4 T cell depletion and progression towards AIDS. What exactly causes this aberrant activation is not clear; however, traits inherent to the immune response seem to be more responsible than the virus itself. For example, there are data showing that the same strain of virus (SIV) can induce lasting but non-pathogenic infection in one primate species (sooty mangabeys) and devastating disease which resembles AIDS in the other (rhesus macaques). It may be therefore instructive to study details of immune response against SIV in pathogenic conditions. B lymphocytes are not spared from overall activation during HIV/SIV infection – in fact, sick individuals display hypergammaglobulinemia, or elevated levels of IgG antibodies. Switched antibodies are generally produced by cells that derive from the germinal center reaction where B lymphocytes interact with the unique subset of CD4 T cells called follicular helpers (TFH). Follicular helpers can be distinguished from other CD4 T cell subsets by their expression of CXCR5, ICOS, PD-1 and the transcription factor Bcl-6 as well as the high level of IL-21 cytokine secretion. TFH have been shown to be crucial for the successful generation of plasma cell and memory compartments. The recent paper takes a look at germinal centers during SIV infection in monkeys that develop AIDS-like disease.

The link:

Authors use immunohistological staining and flow cytometry to prove that PD-1 positive CD4 T cells (according to what we know TFH population) accumulate in germinal centers along disease progression in rhesus macaques infected with SIVmac239. They also correlate the enhanced PD-1 expression on CD4 T cells with remodeling of B cell populations that reside in lymph nodes (the trend is towards reducing naïve subset and enriching memory subsets) and the increased IgG secretion. Additionally, it is demonstrated that CD8 T cells are excluded from germinal centers (both in naïve and infected monkeys). Investigators conclude that the buildup of TFH population during pathogenic SIV infection may play the important role in shifting B cell activation status. Apart from that they suggest that germinal centers could form anatomical sites of potential viral escape due to the lack of CD8 T cells presence (and alleged insufficient control of infected CD4 T cells there).

The experimental setting of this publication is restricted to recording several immune parameters at time points that represent either acute or chronic SIV infection. However, it is interesting because it attempts the systematic documentation of changes in the immune response that may underlie the pathologic immune activation distinctive for some primate lentiviruses. The obvious question for the future is whether there may be any significant changes in germinal center reaction between species that develop pathogenic or non-pathogenic SIV infection.

Hong, J., Amancha, P., Rogers, K., Ansari, A., & Villinger, F. (2012). Spatial Alterations between CD4+ T Follicular Helper, B, and CD8+ T Cells during Simian Immunodeficiency Virus Infection: T/B Cell Homeostasis, Activation, and Potential Mechanism for Viral Escape The Journal of Immunology, 188 (7), 3247-3256 DOI: 10.4049/jimmunol.1103138