Chemotaxis assays for eukaryotic cells.

Chemotaxis is a complex response of a cell to an external stimulus. It involves detecting and measuring the concentration of the chemoattractant, biochemical transmission of the information, and the motility and adhesive changes associated with the response. This unit describes a number of chemotaxis assays that can be used to identify chemoattractants individually and in large-scale screenings, to distinguish chemotaxis from chemokinesis, and to analyze cellular behavioral and biochemical responses. Some of these assays such as the filter, under agarose, and small population assays, can be used to monitor the behavior of large groups of cells; the bridge, pipet, and upshift assays can be used to analyze the responses of single cells.

Integrating conflicting chemotactic signals. The role of memory in leukocyte navigation.

Leukocytes navigate through complex chemoattractant arrays, and in so doing, they must migrate from one chemoattractant source to another. By evaluating directional persistence and chemotaxis during neutrophil migration under agarose, we show that cells migrating away from a local chemoattractant, against a gradient, display true chemotaxis to distant agonists, often behaving as if the local gradient were without effect. We describe two interrelated properties of migrating cells that allow this to occur. First, migrating leukocytes can integrate competing chemoattractant signals, responding as if to the vector sum of the orienting signals present. Second, migrating cells display memory of their recent environment: cells' perception of the relative strength of orienting signals is influenced by their history, so that cells prioritize newly arising or newly encountered attractants. We propose that this cellular memory, by promoting sequential chemotaxis to one attractant after another, is in fact responsible for the integration of competitive orienting signals over time, and allows combinations of chemoattractants to guide leukocytes in a step-by-step fashion to their destinations within tissues.

Chemoattractant receptor cross talk as a regulatory mechanism in leukocyte adhesion and migration.

Leukocytes express multiple chemoattractant receptors that can trigger adhesion and direct their migration. Regulation of such proadhesive and migratory responses must often occur in a complex cytokine milieu in vivo, in which multiple receptors may be engaged simultaneously or sequentially, Here we have examined the interplay between interleukin-8 (IL-8) receptor and formyl peptide receptor (fPR)-stimulation and its consequences for leukocyte adhesion and chemotactic responses. IL-8 has no significant effect on fMLP-stimulated adhesion and migration of human neutrophils, indicating that leukocytes have the potential to respond to sequential proadhesive and chemoattractant stimuli during homing and targeted migration. In contrast, fMLP at > or = 10 nM totally abrogated proadhesive and chemoattractant responses to IL-8, a trnas effect to which the fPR itself is relatively resistant. N-formyl peptides are released by invasive bacteria and lysed cells, and the dominance of the fPR may ensure that signals from these terminal phagocyte targets can override host-derived recruitment signaling through IL-8 and other chemokine receptors. Asymmetric inhibition of adhesion-triggering responses is also observed in lymphoid cells transfected with IL-8 receptor A and fPR, but in this cellular context chemotactic responses are bidirectionally abrogated, suggesting the potential for downstream desensitization of motility programs as well. Cross talk between chemoattractant receptors and their signaling pathways may help target leukocyte migration in the context of complex chemoattractant arrays in vivo.

Multistep navigation and the combinatorial control of leukocyte chemotaxis.

Cells migrating within tissues may encounter multiple chemoattractant signals in complex spatial and temporal patterns. To understand leukocyte navigation in such settings, we have explored the migratory behavior of neutrophils in model scenarios where they are presented with two chemoattractant sources in various configurations. We show that, over a wide range of conditions, neutrophils can migrate "down" a local chemoattractant gradient in response to a distant gradient of a different chemoattractant. Furthermore, cells can chemotax effectively to a secondary distant agonist after migrating up a primary gradient into a saturating, nonorienting concentration of an initial attractant. Together, these observations suggest the potential for cells' step-by-step navigation from one gradient to another in complex chemoattractant fields. The importance of such sequential navigation is confirmed here in a model system in which neutrophil homing to a defined domain (a) requires serial responses to agonists presented in a defined spatial array, and (b) is a function of both the agonist combination and the sequence in which gradients are encountered. We propose a multistep model of chemoattractant-directed migration, which requires that leukocytes display multiple chemoattractant receptors for successful homing and provides for combinatorial determination of microenvironmental localization.